For 1.
As Ethnic Woman pointed out, to get more into that we need more information on the whole mess of the Terra/Eraqus/Xehanort-heart-fusion. Unlike Ven's wounded heart, which only
hides inside Sora's heart, at least Xehanort's and Terra's heart's got fused together, with Eraqus's heart sitting in Terra's most likely like Kairi's heart did in Sora's in KH 1.
For 2.
What
exactly happens is very much unclear at this point. The only tidbit we have for that is from the BBS Ultimania:
-- Ansem the Wise says to Aqua “Everything is born from sleep.” What does this mean?
Nomura : In that scene the ones who are calling for “Sora” are the ones who have already disappeared from the world. But in the world of KH there is no concept of death, that for example they are just sleeping within Sora’s heart. If you can just wake them up, they’ll be able to return to the world, they would be “born from sleep”. That there would be a possibility of this, is what Ansem the Wise is saying. Even Aqua, who isn’t in the realm of light, would be born from sleep.
-- In that case, do the characters we see in that scene have a chance of returning?
Nomura : Yes. The data that Ansem has hidden within Sora is the secret to that revival. When Ansem had become DiZ, and studied the heart and emotions, the results he found he hid within Sora. There are a lot of people who thought that Coded was about finding those results, but that’s not true. The ending of Coded was about finding out that DiZ had done something to Sora, as Namine said, finding out about the data that Ansem hid. What that data is will be revealed in the next game. Two more games aside from KHIII are in the works.
According to this the data Ansem the Wise hid inside Sora (which consists apparently of his research results on hearts, memories and nobodies) is the key to "birth by sleep" aka revival aka "saving the tormented".
How this can be all archieved is still a mystery and shall remain so for a while, as Nomura had to say in the Re: Coded Ultimania:
Speaking of the Birth by Sleep secret cutscene, Ansem the Wise says that he hid his research results in Sora. This complies with what happens in Re:Coded when Namine says DiZ (Ansem the Wise) has hid something inside Sora. Is that mystery remaining unsolved?
Nomura: Yes. Namine added data to the Jiminy Journal in order to convey that the “pain” of those connected to Sora must be healed, but at the point in time Ansem hid the research results inside Sora, he didn’t know that. Accordingly, Ansem’s research results will become the key to healing the “pain” in the future of the story.
So even though Namine and Ansem acted independently, as a result of what the two did, it leads to one thing. And speaking of those details…
Nomura: Since that will likely become the story for Kingdom Hearts III, please look forward to that until then.
The "details" on how everyone will be saved and in what form are according to this a major point of Kingdom Hearts III, to which Dream Drop Distance will lay out the groundwork since in the latest interviews Nomura pointed out that the
main point of DDD will be about Ansem's hidden Data in Sora.
Guess we'll have to wait for DDD then to make some more solid theories on the whole "no death, saving and revival"-stuff.
There has to be a severe catch and/or difficult conditions to be fulfilled in order to work out revivals but nevertheless I'd like to point out that "reviving" someone is not new to the KH series.
We already had it even in KH 1 when hundreds of entire
worlds were revived at the end, not to mention Maleficent and Oogie Boogie in KH 2.
For 3.
I would really doubt that Nomura would waste the characters and their still untouched potential by just dumping them after KH III. I strongly believe that Terra, Aqua and Ventus are going to play further roles in later sagas as Sora's allies and friends, maybe they will even be occasionally playable again.
Regardless in what "form" Roxas, Naminé and Xion are saved even
they might play further roles, even if it's only minor by talking to Sora/Kairi through their hearts.
---
@Ethy:
Nomura didn't say anything about a heart inside Riku.
Here's the interview you're referring to:
-- In "KHI", the robed man, Ansem the Wise's apprentice Xehanort's Heartless (the body of Xehanort's Heartless before he inhabited Riku's body), appeared on the Destiny Islands. Is this because it was Master Xehanort's homeworld?
Nomura: That may be one reason, but if he somehow had some of Terra's memories, we can consider that it might be because he had laid eyes on Riku previously. In terms of the elements of Terra and Master Xehanort, I think the questions of how their power is related and divided, as well as who has whose memories and heart, will become a key to the story* after this one.
-- Is that to say that there's a possibility that within Xehanort's Heartless, the darkness that sleeps in Riku's heart, there might remain something of Terra...?
Nomura: That's also a riddle connected to the next story,* so everyone please use your imagination.