As a fellow engineer enjoyer, I disagree with a lot of what you say here.
Holo actually has quite decent sustain compared to a more "squishy" spec like harbinger, dragonhunter, deadeye, daredevil, or any dps elementalist. Big boomer pulses small consistent heals, you can take turret or AED if you need either more pulsing heals or a save from death, and Heat therapy is going about half the time for a little more. It does struggle more than other classes in solo open world because the class has no built-in toughness and Spectrum Shield is low-impact (also inventions, the protection line, is pretty garbage right now), but in a raiding scenario it's actually pretty good.
I wouldn't turn down a buff to the quickness duration on scrapper, but if they decreased it any more qrappers would just take fewer gyros and more damage skills and continue using all gyros off cooldown. Just take more boon duration if you need more stability in uptime, the meta build only takes 34% which gives plenty of room to take more diviner's pieces. You say the dps is "meh" and "far from what it used to be" when as of right now scrapper has a higher bench than it ever has (36k) while still bringing lots of self barrier and group superspeed.
I will say I really miss the 15% damage to barrier conversion in PvE, because since this change scrapper's got terrible sustain in solo situations and does better against open world champs on a condi build than it does on power. This is honestly silly on such a power-focused spec, and I wish it had that barrier buff back (my suggestion would be if you took the top grandmaster that no one uses you get a 200% barrier buff instead of the current 15%; it wouldn't be as good as it was but it would be more balanced).
Regarding mechanist, I think this is a good change. If people want to do good damage on engineer, they can go scrapper, holo, or core, all of which do over 35k. If they want to afk while doing their daily strikes, they can play mechanist, and still probably get near top damage in the average PUG. I'd be interested to hear what your other ideas are for changing the public's opinion on the class, but I think specifically nerfing their dps when the player is at range and has abandoned their jade tech pal is a good way of doing it.
Cheers!