Hello! Dev of Summit for Lemmy here. Summit actually supports switching accounts while keeping the same post open. Of course due to defederation it is not always possible to open a post on another account but it tries it's best.
I don't seem to have that issue using Jerboa. I am on Lemm.ee and have no issues commenting on Lemmy.world, lemmy.ml, etc posts. The only case where I could see not being able to is if there was defederation which the admin of lemm.ee does not want to do, even with the genocide deniers of hexbear.
Also it can be really hard finding small communities from a different instance, a lot don't show up or aren't fully synced so some of the posts don't show.
Hopefully this is going to be fixed but I think it's limiting the growth of more niche communities.