“Allowed Hosts” is white listing btw. You should leave it blank if you want more than just lemmy.world and lemmy.ml
And I wouldn’t say it reduces the load on those sites. It would kind of add to them in a way if you post from your instance or sub from your instance solo. If it’s just you on your instance, it’s barely a blip. May save them some photo hosting if you post.
SOME of the out of sync upvotes/comments are fixed in 0.18.1