Should all archetypes be able to solo?
Absolutely. I solo'd this game a lot on Live and I think that's one of its greatest strengths is that it is very solo friendly. Now the level that people can solo at that's a whole other question.
Should all archetypes be able to solo at +4/8?
I'd say overall that fighting at +4/x8 shouldn't be without difficulty, no matter the archetype, builds, IOs etc. The limit of difficult should feel like the limit unless a higher limit can be established.
Is there a difference in performance expectations for content below level 50 versus level 50+?
Starting at lvl 35 the game starts to get super charged with T9 powers and epic/patron pools. This is part of the natural game progression where that group of 3 minions isn't anywhere near as hard as it was at lvl 1. Obviously, once you get Incarnates and Purple IO sets that power equation is going to change again.
Should players be using Invention Origin enhancements by a specific level or range?
Generics? usually starting at lvl 22. Nothing but sets? Not until 50 I'd think unless you got money from your alts or someone generous. Most normal content should be soloable at +0/0 with an SO build at lvl 22+
Do players who can solo +4/8 at level 50 have an outsized impact on teams or other players? Maybe this is as intended?
Absolutely. However, it takes awhile in terms of getting IOs and Incarnate powers and that really is a reward of those systems. I think being stronger than other players is intended but absolutely eclipsing other players (at the same or similar level) I don't think was planned by the original developers or Homecoming team. How to deal with that or if to deal with that is beyond me, sorry!