I agree in many aspects. In more detail:
1. FIX THE HUGE NUMBER OF BUGS! First thing on the list. These are famous and need fixing, I wont go into them here.
Hmm. MC actually is not as bugged as many people claim it to be. (Mind you, i am speaking of BUGS here. Not of bad design decissions. Of those it has plenty.

) There are some, but some people here still base their opinion on how terrible it was at launch and never tried it later on.
I used it a lot till about a year ago, when some of my casually playing friends still logged in occasionally. Since start of 2019 i only did two or three MC sessions, though. But during the time we used it a lot, actual bugs were not that rampant. The real problems were in the core design instead.
2. ENCOURAGEMENT FOR CMDRS TO OPEN THEIR SHIP TO MULTICREW. There are few crew slots available, it is very hit-or-miss whether you can join a MultiCrew session at all. Mostly a miss. There needs to be more incentives. Could be extra Arx.
Not only this, but also better rewards for those joining up. And the very first fix would even be an extremely simple one: remove the PUNISHMENT for joining a higher ranked pilot. I am not even speaking of an incentive to go MC here, but only to get rid of active punishment.
It's especially ironic: the game punishes my friends for me continuing to play and increasing my combat rank. The higher my rank, the less they get, despite doing the same thing and performing all the same as before. This is an extreme case of anti-social game mechanics. It's hard to make it any worse. I do not know who on the FD team thought that such negative feedback would be a good idea, but i can say that he really should spend some time to learn the very basics of game design.
Where i disagree is using ARX as reward. Pushing and promoting any game activity and mechanic with ARX is a bad idea. Things have to be made interesting by in-game mechanics and rewards. Anything pushed by an extrinsic reward like ARX will be purely seen as grind. People will do it while not enjoying it and hate it even more than before. A classical negative feedback loop. Nothing positive can come out of that, it should be avoided at all costs.
3. MAKE IT OBVIOUS HOW TO INVITE SOMEONE TO A MULTICREW SESSION. I have seen a lot of CMDRs complaining they could not get people they wanted in their crew and it turns out they were just relying on the automatic instancing to do it.
Hmm. I think you're right there. Once you have figured out how to start a MC session it's not too complicated, but if you never did it yet, you have to look around a bit at the start. Intuitive design is something different...
4. CREW SLOTS TO BE SET AS OPEN BY DEFAULT. One big thing that would massively help make MultiCrew more popular.
Hmm. Never affected me. I normally use MultiCrew with SLFs on board. The gunney role is not enjoyable. We tried that once, it just wasn't fun. It felt like both pilot and gunner had merely half a ship available. For the very reason that this is exactly what we had: half of the ship we would have had in a wing setting. With my friends using my SLFs, things felt very different. Everybody had a complete vehicle available, with sufficient weapons, each able to enjoy the game. It's the one way tht MC actually is fun.
5. WINGS WITH MULTICREW! What a massive oversight. So many people would love to have crew+wings but it just cant be done atm.
Absolutely! So very much this! Unfortunately here FDs lack of foresight shows. Wings were designed without even a second of thought about MultiCrew. So when they a little later started designing MC, they realized that they designed something completely incompatible to what they want to do now.
I so much would like if both things could be combined. But based on how things work in wings and MC, i guess they'd have to rework either ground up to get this done.
That being said, one good start would be to improve the handling of wing missions when using MC. One of the things i liked to do with my friends was to grab a wing assassination mission, share it with them, then go for the mission. But due to the incompatibilities of Wings and MC, you have to first wing up, share the mission, split the wing, join for MC, fly the mission. That's extremely cumbersome and no fun.
Merely the small upgrade that the owner of a ship in MC can pick up wing missions and share them with his crew would go for such a long way. And this is something where the implementation should be possible with reasonable effort.
6. OPTION TO OPEN 1 SEAT ONLY + other fine-grain control options. In 3 seat ships both seats open up and there is not much control over things. There aught be more fine-grained control over who can sit on your ship, and also what they can do while sat there.
Hmm. Maybe. It's not of any importance for how i use MC, but there might be a use case for it. But i guess when opening your ship for random people, the thing should go the other way: it should display which roles (gunnery yes/no, number of SLFs) would be available.
Would pilots actually open their ship for MC, this info would be useful for others looking for a ship to join.
7. OTHER CMDRS TO CONTROL YOUR SHIP while you go in a fighter. Or go to the loo. Or watch TV on a long voyage you are doing together. For explorers especially I could see how this would be great and am surprised it was not done. Could have been done really nice with expeditions and 2 CMDRs name being on things(as an option for primary CMDR to toggle on). Its a great way for friends to journey together, esp if 1 can control navigation (galaxy map etc) and other flying the ship.
I see the point on the exploration side. I wouldn't give control to my ship to anybody else, though, as i see options of abuse but also the risk of bad misshandling. Mixed feelings here.
8. MULTICREW AVAILABLE IN SOLO. Open MultiCrew for solo players. There is no risk of ship loss as a MultiCrew crew member.
Hmm. From the point of view of joining MC, this sounds like plenty of work for little gain. I mean, if i would want to join some ship it doesn't matter if i have to go to solo, open or a PG first. You log in while your ship is docked, zero risk, then go join a ships crew.
For somebody owning his ship to MC this might be a bit more of a thing. But you bear risk with other players, even if you officially are in solo. As you mentioned yourself at point 10. Not much gained with this, in my eyes.
9. MORE SHIP CONTROLS AVAILABLE TO CREW. This has to tie in with more fine-grained control of what/who is allowed to sit in the crew seat. Right now MultiCrew is useful only for combat, and perhaps(one could argue) mining. It has so much more potential. Exploration was mentioned above, galaxy map access would be needed for a navigator to plot the course, pilot to fly the ship/do the scooping. Srv control by crew member. Fighter to scout planet + Srv. Many things would be great to do with friend.
A first step would be to simply allow the FSS to be used when the ships throttle is not at zero. More of that many people (me included) suggested in FSS connected threads, no need to convert this thread to this topic. But it would really make not only exploration, but also wing missions, a bit more enjoyable.
The mere use-case: i already was successfully interdicted (throttle at zero, so you submit immediately) while using the FSS to find my mark. Luckily the one interdicting me was my mission target, but still: i hate to not know what's going on around me while using the FSS. In a MC setup, the pilot could watch his back, move to a good position, etc. while the crew uses the FSS to spot the mark. A clear improvement.
(And sorry for mentioning the FSS. There's a significant risk now that Burke turns up here and tries to turn it into yet another anti-FSS threads. But it's one of the things which really could be upgraded a lot for MultiCrew use. )
On some other things you mentioned in this point: it would be great to have them. But FD designed themselves into a corner. They built things without planing ahead, then realized that what they build and rolled out is not compatible at all with other things of the game. Getting the things you suggest would be absolutely great, but it would also require a lot of effort to tear down and rebuild a number of things they made for wings and MC. I doubt FD will ever do that.
10. PUNISHMENTS FOR 5th COLUMNISTS/TROLLS. Opening ship to someone is an act of trust and deliberate sabotage is not an acceptable form of gameplay. I have no idea how this could be done. But I dont accept MultiCrew treachery as something that is valid gameplay. People know me I am pretty liberal what I find acceptable but this is not.
/sign
Absoultely this. It's why i never open my ship to people i don't know. I mean, with my friends accidents also happened when they started out. They got me wanted and with some notoriety when they were new to the game and piloting my SLFs. But with them, i then just established one rule: the next one to get me wanted pays my beer the next time we go out. (Bad luck for me: from that moment on, they always watched their targets, never got me wanted any more. )
Unfortunately this is nothing i could establish with strangers on the internet which just randomly join my ship. So all the risk would be on me, with exactly one way to control or prevent it: by not using MultiCrew at all.
That's not encouraging to use MC. It's bad and should be changed.