1. Carol's upcoming series will not last, either due to low sales or editorial mandate or whatever.
2. As I am sure many people on this board saw the Avengers movie we know that the upcoming cosmic threat is none other then THANOS. Who was it that originally stopped Thanos all those years ago when he had the cosmic cube/tesseract? CAPTAIN MAR-VELL So, let's presume the Avengers sequel sticks with Thanos...that opens the door for Mar-Vell, Drax and Adam Warlock to appear also though I suspect they wouldn't use all three.
If Mar-Vell is going to be in the sequel then since Marvel wants their comics to match up with the movies (reference the shameless removal of classic Nick Fury from mainstream marvel to be replaced by and illegitimate son named Nick Fury Jr. and his pal Coulson) then Mar-Vell returns in the comics.
3. Those plants were more then likely the alien race called the Cotati who also inhabit Hala, the Kree homeworld. They are telepathic and have been known to circumvent death, reference the original Swordsman. It is possible that the Cotati, combined with a residue of phoenix energy within Mar-Vell, will rejuvenate him. However there are two problems with this:
Problem 1: the phoenix rendered his cancer inert but the deadly cells were still there, remove the phoenix energy and he was pretty much instantly dead. This time the cancer cells MUST be completely cleansed and purged from him, not just rendered INERT as that is a cheap plot device to kill him again.
Problem 2: the Cotati elder reanimated the Swordsman's body but the Swordsman's spirit was not in it......so can they recall Marv's spirit back from the hereafter?
Of course it doesn't matter. Marvel has basically decided to screw with Mar-Vell fans with these fake revivals or cheap skrull knockoffs.
Your points are very logical Iron Man 007...specially with the futur Marvel movies, Mar-vell is too useful for Marvel: for to introduce Carol Danvers, but also for Thanos and also the Kree/Skrull war....
But I think with this plant story, his return will be just for to link the comicbook with the Movies, and I think it's the biggest problem...I really see Marvel try to do "the Death of Captain Marvel the movie", because the story is very special and I think Marvel would try to take the opportunity to make a real superhero dramatic movie...and it will really a horrible thing to do for the Mar-vell fans...
I hope the plant will be the Cocatis, and they will help Mar-vell to comeback for good...I can easily see them to do that for to comeback to Hala and help the Kree (despite their conflict in the past) about their recent problems (or maybe find a solution about the genetic kree problem)...Now, if it would be Mar-vell with his spirit or not...I guess everything is possible, specially in the short time after his phoenix ressurection...but I will be very happy if he is "just" present physically with the help of the Cocatis.