1a. These are not the best tool to find communities. In particular sub.rehab is curated rather than being a full Lemmy search tool

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 (say lemmy.world) will only show what that instance can see i.e. communities and subscribers from federated instances

1b. 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