Can you self-host, or are you looking for another online service? Facebook Groups is basically a forum when it comes down to it, and any forum software can do what you're asking. I really like Discourse. You can self-host it for free (well, whatever your server costs), they'll host it for free if you're an open-source project, or if you're a legal non-profit you can get 50% off their hosting for $25-50/month.

Comments are pushed out by the community's server, they're not pulled in by yours. So if you're missing comments from communities hosted on lemmy.ml for example, it may be that lemmy.ml is overloaded and not sending out comments to the fediverse properly.

The other common issue with missing comments and posts is misconfigured language settings in your profile. You need to make sure at least Undefined and English are both selected, lots of people only have English selected which will make a lot of posts hidden.

I would describe Apollo as an accessibility app in the sense that the regular Reddit app is unusable.

"The content" on all Lemmy instances is the same. There is no account migration, but you can just sign up on lemmy.ml. If you already had an account there and you want it back... I don't know if it's possible for an admin there to restore it, you might have to get in touch with them.

Downvotes just don't work inside communities hosted on lemmy.one. They might work on your own local midwest.social instance, I'm not sure, but if you downvoted my comment here nobody would be able to tell on lemmy.one, and nobody would be able to tell on other federated instances like lemmy.ml or beehaw.org, because lemmy.one simply would not federate that information to them.

You might want to check out !lemmy_support@lemmy.ml for asking questions, and !lemmy@lemmy.ml for reporting bugs and requesting features :)

Mods available to be added?

Not sure what you're asking here? About creating communities (subreddit equivalent) and adding mods for them, see my comment here: https://lemmy.one/comment/536

You can collapse comments, it's just not really intuitive, click this button:

*removed externally hosted image*

No downvoting on lemmy.one:

Downvotes are disabled on this instance, because it is a very small community. If you see something against the rules, report it. If you see something you don’t like, go find something you do like and upvote that instead :)

I may consider changing this in the future.

If you have more questions about this instance, lemmy.one, generally, you can also ask at !meta.