To inject Rhea you need Falo's save editor (you can get it on the front page or in the post right above yours) and of course your save game. I'm not sure if it'S available for atmos, but you can dump it with "checkpoint". also you need rheas character file. there are two versions of her (link on the front page) and I tested the one on page 150, which is her "Saint" Class Version (though stat wise the two classes are identical, at least from what I found).
So to inject Rhea you open your save file in the editor, click on the roster tab and then choose an unused character slot (I somewhere read that you should pic slots from number 51 upwards). Now you can click "import" and select the rhea.character file you downloaded, don't forget to click "save changes". The last thing to do is wirte the save and you're done.
Regarding your questions:
1) She can change classes, I didn't pay attention to the spirte, so not sure about that. You can read her stats if you click on the "test" tab in the save editor and then choose rhea (her character is number 0036). About her abilites/combat arts/spells: I don't think she learns new abilities or combat arts, she just has some by default (you can check them in the roster tab. you can edit them there as well). What I found out is that is able to learn spells (some, already learned by her are also listed in the roster tab, but she learns two more in reason: Ragnarok at rank A and Agnea's Arrow at rank A+).
2) Since I haven't done a full playthrough with her I'm not entirely sure, but I tested her in a normal battle which worked fine.
3) You can just add her, no need to overwrite anyone. She then appears normally in your ingame roster. One thing I should point out is that I could not teach her like the "normal" characters, so it's more difficult to acutally train her (then again, the poster I got the .character file from said he was able to teach her normally, but I didn't look further into that).
Right, that's it, I hope it helps!