So I've noodled it, and want to retract some of my earlier gripes. I'd managed to forget, in the sheer mass of story washing over me, that past-Nebula took our Nebula's place and used the quantum thingie to pop out of that now-alternate timeline to the "main" one and then bring Thanos' ship through after her. So my complaint that the alternate timelines were impacting the main one is null. The others didn't, and the alternate Thanos one specifically employed the BS quantum tech to cross the streams. No causality problems of Nebula killing that version of herself, or Tony dusting Thanos and his crew. Nifty. That's a huge niggle resolved for me.
Which also resolves part of the other big complaint. Steve. Now, I don't have a problem with him going back to nineteen-forty-whatever and taking up with Peggy. It's an alternate timeline. He probably did, in fact, thwart HYDRA's infiltrating of S.H.I.E.L.D., maybe even helped found/run it with Howard and Peggy. Probably went and rescued Bucky at earliest convenience. May or may not have stepped in other places where he felt Cap was needed -- Cuban Missile Crisis? Berlin Wall? Kennedy's assassination? Apollo I? Challenger? Heck, maybe even prevent Peggy's Alzheimer's (foreknowledge of Extremis?)... Again, no conflict with the main timeline. And for people who feel he abrogated his sense of duty, there are plenty of heroes now, especially post-unsnap. He's not as crucial. And he brought Sam that shield from his timeline to replace the one that had gotten destroyed in the main one.
That still leaves two and a half questions...
Returning/restoring the Space, Reality, Power, Mind, and Soul Stones. Power is only a little tricky, given how little we know about the Orb and opening it, and the containment field and how to interact with it. So that's minor. The housings for Space and Mind were destroyed to retrieve the Stones' essences, and I don't know that we have the capacity to re-create such artifacts, or who would. And Reality, we don't know how it changes states. We didn't see what Thanos did to get it in his gauntlet in IW -- he already had done it when the Guardians got there. And we didn't see what was done after Rocket extracted it from Jane. So maybe we already knew how to undo it?
But Soul... (this is the "half) I still feel like there's a whole lot around returning that that is relevant and needs to be seen. How he reacted to the Guardian, that'd be a bit of a moment for Cap. And the huge question of whether returning the Soul Stone gets back whoever was sacrificed to get it. If not, that's a pretty BS deal.
Last niggle is that, while sharp-eyed viewers noticed Steve grabbed more Pym Particles in 1970 than were needed to go back to the future, I haven't seen a suitable explanation as to why Old Steve was sitting on the bench, rather than returning to the platform where all of the other quantum-travelers returned to after their jaunts. That seems to be the crosstime gateway. I'm gonna guess that he timed it to a moment when he knew the platform was unattended, but I feel like it shouldn't be 100% on the audience to work through the logistics. The film should give us a little more to go on.