True. I concede to your point. However, you are able to skip requirements.
I never said you couldn't - there have been multiple bugs related to unlocking engineers over the years. They've been introduced, squashed and unintentionally reintroduced. I believe FD got rid of several unlock requirements as it solved the problem once and for all, and improved the new player experience.
Case and point here: You can complete other engineer's reputation than finish the Liz Wilder reputation mission that returns the engineering invite which consequently completes all the Lei Cheung requirements that allows you to immediately visit his workshop to build reputation; and this in turn returns an invite to another engineer.
I assume you mean Liz Ryder. If that's the case (and you aren't talking about a brand new, unknown engineer), then I can say quite confidently that something different from what you describe is happening - I only unlocked Liz Ryder today (on my newest account) as I decided to engineer some high capacity dumbfire missiles for my Cobra Mk. V.
The Cobra V already had G5 reinforced shields on it as I'd already unlocked Lei Cheung. I didn't have to donate 200t of gold, nor did I have to trade in 50 markets:
This indicates that certain requirements
are skipped, but that it is not related to your relationship with another engineer, at least in the case of Liz Ryder.
Another case and point: The Dweller can be unlocked and his workshop made available after accepting any cargo delivery mission than abandoning the mission to turn the goods into stolen goods, and you go and visit his workshop to engineer your power distributor with a r5 blueprint.
The same lack of requirements, stemming from a failed bugfix, means that the Dweller is unlocked by default. The in-game descriptions for the engineers still refer to the unlock requirements, which is unfortunate.
I would note that none of the above facts support the claim that Pailin and Sedisi can be unlocked via only your relationship with Qwent. Speaking of whom, ...
I could name other case and point examples. Marco Qwent offers nothing than another pinned blueprint. He is the steppingstone to the one other engineer to acquire the other final 2 ranks for Thrusters, whereas The Dweller and Hera Tani allows you to apply rank 5 blueprints to the PP and PA.
Yes, that is the unlock flow - you need to gain reputation with Qwent in order for him to pass you the information on Palin and Sedisi. I last unlocked Palin and Sedisi quite recently - within the last four months. I would note, however, that even with Qwent at maximum rank, I still had to travel 5000LY to get the invitation and collect 25 sensor fragments to unlock Palin.
As for Qwent offering nothing more than a pinned blueprint, that's what they all become in some fashion or another. Engineers are a bit like Pokemon in that sense. The special effects are nice, but they're not game-changing upgrades, especially in comparison to the power increase offered by just the pinned blueprint.
If you have evidence that you don't need to do the 5KLY trip, I'd like to see it. It would mean that I can change the advice I give to new players, and will save me time when I next start a new CMDR.
[Edited] Is it supposed to work that way? I do not know except that it does help new players get through unlocking and building reputation with engineers easier. At the end of the day, the ability to skip over requirements helps those players start out in Elite Dangerous. Plus, it's ridiculous to ask a new player to make a 10,000 LY round trip into the black with minimal or no FSD engineering: That's like asking them to spend their game sessions doing 230+ jumps (43 Ly jump range) for an engineer requirement? No other engineer imposes such a huge inconvenience on new players.
As I noted above, I've heard that some of the entry engineers unlock requirements were basically switched off due to bugs. I agree that it really helps new CMDRs, and I suspect this is why it was left as it is. The fourth time you start an account, trading in 50 markets and donating 200 units of gold gets really boring.
As for 5KLY, I think that on the one hand it does encourage players to try their hand at exploration, and the famed payouts for exobiology are only available when you start getting first footfalls, which requires at least a couple of thousand light years of travel. This is not a bad thing, as it means new players can basically get themselves the billions they will need to buy and outfit their fleet while also unlocking Palin and Sedisi - in that respect, it's actually rather convenient. On the other hand, exploration is not for everyone - I personally don't enjoy it.