1(a) These are not the best tools to find communities. I believe sub.rehab is curated rather than being a full Lemmy community search tool (i.e. good for finding the few official migrations)

To find communities, I would use lemmyverse.net

For example, you can use this search to find Android communities

If you set your instance using the home icon, any links will open on your instance so you can subscribe to them correctly

It shows similar subscribers counts etc to your first two searches, because it is an independent tool, while searching from your instance (lemmy.world) will only show what that instance can see i.e. communities and subscribers from federated instances

1(b) Each of these are different communities, with different content, moderation, and members. Sometimes there will be duplicate content and cross-posting. If you think they are so similar that the duplication is annoying, just pick one, otherwise subscribe to both

I understand that the way cross-posting is presented is being worked on for future Lemmy releases, but honestly duplication is not that bad at the moment except during large news events

2 Where are you getting links to Lemmy content that you are trying to open? Once you have subscribed to communities of interest, you may come across links to other communities in that content. There are bots that often 'correct' instance naming in Lemmy content to use the ! notation, so as long as you don't block them you'll get clickable links

Otherwise, yes it is currently the case that /c formatted links will need to be changed to your instance or ! formatted - I'm not aware of an extension to do so, but I presume some of the apps and front-ends will do this for you

The concept of "official" community doesn't make any sense and should be ignored despite what some might try to sell you and feels as if the proponents of such wording are implying that there's some trademark on Reddit community names, which is again stupid. A community having a Reddit mod or two isn't necessarily more "official" than another that has mods who were never Reddit mods, it's a stupid argument advanced mostly by Reddit mods who want to preserve the same role on the Fediverse. Being a mod on Reddit is not a spectacular achievement that you should put on your resume. r/neovim tried to make their "official" community on the Fediverse instead of just telling their members to join an existing one, pushing the same arguments, but fast forward 3 months later their instance is dead (literally, the server is offline), because neovim communities on sopuli and programming.dev without Reddit mods succeeded instead.

And you are right in that the top communities right now are not modded by Reddit mods (and hence hilariously not designated as "official" by those who still subscribe to the Reddit ideology).

Also on the fragmentation "issue", it's a false problem to begin with. Lemmy is not a Reddit replacement, it's multiple Reddit replacements, some specialized and some very general, that are federated together in the sense that you can interact with the communities of some "Reddit" from a different "Reddit". Think of it as a big network of web forums, except that you don't need to create an account on every forum.

More choice adds maybe to the confusion of people who still wrongly expect Lemmy to be one single big Reddit replacement, but more choice is still healthy and I'm glad there are many communities out there even around seemingly similar subjects. One example is that I don't like the automated episode posts in [email protected] (but many others like them), so I'm glad to still have other anime communities like [email protected] despite it being smaller.

More choice is better and you don't risk having mods with too much power and be the only ones dictating how you should spend your time on Lemmy if their communities were the only ones around particular themes.

How you should go about Lemmy: Forget about trying to reproduce the exact same experience as on Reddit. Join an instance that you vibe with (also be wary of the fact that small instances, while potentially fast, have also the associated risk of maybe shutting down someday, as some have already died, and on the other hand big instances while they may have big resources they are usually bigger targets for DDOS attacks, so make your choice while taking that into account). Look at c/all. Look at the profiles of the frequent posters and see where they are active and join those communities if you vibe with them. The rest will come to you by itself.

Also for discovery, use https://lemmyverse.net with the "smart ranking" enabled.

If you have no idea where to start, you can subscribe to generic communities like asklemmy (both on lemmy.world and lemmy.ml, both are really good), [email protected], and of course [email protected] (👀).

Too many opinions without the bigger picture, but there in lies the problem with adoption. Your questions have been answered already with the few detailed replies. Definitely don’t assume there’s some “official” community that’s automatically better than all of the others.