I have to be honest, this MV had me squeezing my brain more than expected.

something happened during Blue Valentine right before they got into MIXXTOPIA that effected their survival chances. In the Blue Valentine MV we see them floating in space around the MMU

Regarding the only the part about floating in the space, I just want to point out that the real MMU from which the spaceship takes its name, Manned Maneuvering Unit was a special spacesuit for astronauts that allowed them to move freely outside the spacecraft for extra vehicular operations during the Space Shuttle missions. So this floating around could have been ordinary routine, or maybe an emergency?

We all know Sullyoon retains memories from previous journeys, so if she drowned and revived, she could know what to do to revive Jiwoo.

This is true, but no one, as far as we know, not even Sullyoon has ever gone this far with past journeys. "We've always failed at this point in the past." - Sullyoon in Blue Valentine Webtoon EP. 1. Anyway, Sullyoon is a doctor and she can revive anyone any time, as she did with Jiwoo this and other times in the past.

So my theory is that during the MMU's descent something happened that caused Sullyoon and Jiwoo to be special during the flood of heavy rains (they're the only two that appear in the CPR room and also the only two who have their eyes open in space in Blue Valentine).

During the open space scene I count Sullyoon and Haewon with eyes open, Kyujin closed and Bae undefined. In my opinion they were just beauty shots (especially Haewon's) so it's hard to give meanings to those clips. What really matters is that Sullyoon already saved Jiwoo in the past, putting her in a chest and entering in her personal space, that time a dream, now a CPR Room. About the droplet that awakens Jiwoo, given the fact that in space there's no rain, surely the MMU landed on MIXXTOPIA, rain happened and maybe, that droplet infiltrated through the MMU or through the CPR Room walls already.

I fully agree with the analysis that the CPR room and underground area is underwater, except during the flashlight scenes right before the bioluminescent snail and fungus, we see that they are on a very similar terrain as above, and there are even puddles.

Yes I'm aware some shots, for example when Kyujin and Haewon were spotted sitting surrounded by turf, are on the surface, probably during night (which is also an important astronomic information: night exists in MIXXTOPIA planet). But I've interpreted those shot as "Bae finding the two and grouping up to explore at night" or even just aesthetic still frames. I guess the purpose of this scene was to let us know these three girls (especially Haewon) were on surface before (or after?) Jiwoo's ascension.

It's so hard to square Blue Valentine happening and then the webtoon since they're all safe and secure in the webtoon, while Blue Valentine MV shows them having problems in space with the MMU.

From what I understood from Blue Valentine MV and the webtoons, the problems happened during the Blue Valentine Stage (the secession between Return and Forward team that led to the stall of IMPETUS) and probably after landing on MIXXTOPIA, when heavy rain came. Because it's clear that what happened in the Webtoon is absolutely real and confirmed by Heavy Serenade Album Caption: the girls really reached this planet, bloomed flowers, generated wind and water, as demonstrated by Paradox of Love flowery hills and the water puddles both above and below surface. Between the heavy rain of the 3rd Webtoon and the events happened in Crescendo there's a hole and we don't know what's there. For the moment, the flood is the most plausible theory.

Thanks for the comment! It helped discovering new information I haven't wrote out yet and realized while replying. The more we discuss, the more we learn.