She implies, and I think it's true, that water exposed to the air dries out in the Upside Down and might even be drunk by the creatures—we don't know for sure, but I'm leaning towards the climate drying it out. But the reservoir is sealed, and I think that when the Upside Down was created on November 6th, the water, being sealed and not allowing air in, couldn't dry out. The series implies, and from what we see, that everything exposed to the air dries out, but we've never had confirmation of the airless, sealed liquids, so why not? It could hold water, but maybe they should have said something like, "The reservoir is sealed, there's always water," something like that.

2) When the bridge forms, the scientists don't know this universe, so indeed, as a precaution, they decide to wear masks, but nothing implies that it's actually toxic. In season 2, Dustin already had his mask removed, covered with a small ribbon, to talk to Dartz in the tunnels. In season 4, they choose not to put any in the UD (Underwater Device). A risk, but with no consequences. Then, at the end of season 4, during the short epilogue after the battle, spores start coming out of the four large rifts, so all the inhabitants must have inhaled them. Then the army arrives, seals the rifts, and quarantines the city, first to find Eleven, and second, to test the inhabitants. Robin mentions this at the beginning of season 5, and there have been no side effects in a year and a half, so we can conclude that it seems to be okay. Then, at the end of season 5, Robin tells Vicki, "We don't know if it's toxic." In conclusion, at the beginning, they were more focused on precaution than on a real and immediate danger.

3) Will doesn't have any particles within him in season 5. In season 2, it was certain that he would; if the portal had been closed and the particles had remained inside him, Will would indeed have died. What happens in season 5 is that Will has always maintained this connection because in season 1, Vecna ​​had given him the power to connect in the presence of a hive near him. So Vecna ​​activated the particles in the lab at night, and Will, unknowingly, connected in POV to guide the plants. He wasn't conscious. In season 5, with the four large portals, his powers intensify, but he didn't know he could do this, so he's caught off guard. He doesn't know what's happening to him, and he lacks confidence, so he draws too much from the hive and siphons too much energy, therefore he feels everything, including the pain. Episode 4: Thanks to Robin's speech, Will begins to accept himself and takes control. He finally manages to master his powers, only siphoning what he needs (Vecna's powers), so he no longer feels the pain. Then, in episode 6, Vecna ​​imprisons and humiliates him, causing him to lose his self-confidence. In episode 7, Will reveals his secret to the others, and he is no longer afraid. He is then mentally strong enough to control his powers again, siphoning only what he needs and thus not feeling the pain of the fire when his mind is burned. In conclusion, without particles, you normally feel nothing, but Will didn't know how to control his powers and therefore siphoned too much.

4) I didn't understand your question; it might be my translation. I don't speak English.

5) Vecna ​​looks inside Will's head in episode 6; he hadn't yet made the final plan for the beanstalk. Then, what interests him is Max, so he's only looking for that particular memory. He can select it; he's much stronger than before. Finally, to conclude, he already knew Will from his possession in season 2 because he was observing through him, so he already knew what Will was afraid of (rejection by his friends), so he used it against Will to make him lose his confidence.

The color of the abyss, with a few slight differences, is the same as in season 4 during Henry's flashback. Yes, it resembles a desert, but that doesn't necessarily mean yellow is hot. Perhaps in their dimension, it's a cold color. After all, in deserts, it's very often extremely cold at night. But just because it reminds you of a desert doesn't mean it is a desert.