So what is the difference if you can unload the cargo at the originating station as a temporary solution to switch ships etc? you still have the mission, and it is still running, regardless if you actually have the cargo in your hold or not... This would just be a small extension to how the wing version of cargo missions works, as opposed to how the old single style missions works where it refused to let you accept the mission if you could not load the mission directly onto your ship, wing missions, do not automatically load your ship, and that of course have had its fair share of CMDRs that accepted mission, flew away and forgot to load the cargo.. So having that same mechanics for solo missions would allow CMDRs to accept missions in ships that cannot hold the cargo, and also do part deliveries of a bigger haul, and with a small adjustment that you would also be able to unload the mission cargo you have already loaded back to the originating station would not make to much change.
This would not change the game in a significant way, it would instead make things behave the same regardless if it is solo cargo mission or a wing cargo mission, and the only difference here would be how many players can participate in completing the mission. And the only change would be the option to actually return cargo, and that is probably just a minor change to the current behaviour we have in Wing cargo mission. And like I have already posted, if a CMDR wants to utilize this option, it has to be at the originating station... so it still requires some commitment from the player, and they still have to remember to complete the mission later.
This would also allow you to lucrative accept cargo mission in any ship, and then return with a suitable ship to comple the mission, and if you forget about it, the mission will fail with the normal result of failing to deliver the cargo on time.
As for your "garage" example, we know the cargo is tagged, if you loose some cargo to piracy, you cannot simply go to the market and buy some replacement cargo of the same sort, so you can think of this as the cargo is tagged and sealed in place, now you cannot access the cargo without breaking the seal. which they will know, so you cannot simply unload the cargo into your garage... This is how we have arranged transportation of gods that is not allowed to be tampered with during transport, we seal every door to the cargo hold of a truck, and we verify the seals upon arrival... so that is how I like to think how the mission cargo works in Elite... you are not allowed to remove cargo that have been loaded onto your ship, only the mission giver or recipient is allowed to do that.
[OT]
I have a different opinion on the the time limits. I do not think it is prudent to have time limits on mission to be real time calculations, I would prefer to have it count the mission using in game time instead, ie, it would could down the mission timer when you play, that way, it opens up for FDev to give out lucrative missions that have a very narrow time limit, so you have to commit to do these. And if a mission requires a specific state, for example massacre mission during war, if the war is over, you can obviously not complete the mission, and I do think all those mission that requires a specific state should auto fail with no penalty if the state has changed... Compare this to how you today can pick up massacre missions just before the tick and then after the tick, the war is over and you can no longer complete the mission, so you will fail the mission, despite that you did not even get a chance to actually complete it...
