1: yes
2: no
3: Wilhelmina is fine, don't know about the students, maybe they'll study in the astral observatory, or they'll take over the clockwork palace and study there
4: i'm not somebody from frozenbyte but i hope so, in trine 4 there's bonus content which tells how the story of trine 3 ended, which may mean we'll never see how it ended ingame, also many aspects seem to have been retconned, like the artifact appearing just before the final battle, it was destroyed during T3 and the heroes gained trine's powers, which i thought implied the artifact was no more

1. Yeah, people were cheering when they got off that airship wreck. Plus, the Astral Observatory wizards could have helped in vouching for them.

2. In the Discord server, one of the devs did say they have been destoned by the Observatory wizards. This was just not included in the ending to save on production time.

3. Safe to presume WIlhelmina is safe since the heroes retrieved her soul orb thing. And the educational fate of the 9P students is touched upon in the main menu's "extras" that you unlock by collecting all the tidbits. You find Owl and Nim in the Red Rose Inn talking about how they're both out of a home and a job respectively. On the Discord server, a discussion with one of the devs joked that graduation would be difficult seeing as the Academy literally no longer exists, so the students' studies might be on hold for now.

4. I personally doubt they'll complete the game itself, but they might fill in the story blanks through lore books in future games?

I feel that Trine five will have a sequel, not trine 6 but another trine 5 like melody of Mystery,
because trine has a problem to be unfinished. Like, Trine 1, they beat up Sarek but I didn't see the kingdom become better.
Trine 2 was fully finished, but we had no idea where Isabel went. There's a theory she joined the council in the Kingdom of Trine.
Trine 3 left off on one shard of trine and an extremely hard boss.
Trine 4 was kind of finished, but we had no idea what Selius did afterwards.
Trine 5 is totally unfinished. First of all,
1. Didn't the red rose inn get trashed by the soldiers?
2. Why the hell did Sunny blow up the academy? What are we gonna do with that?
3. What happened to Whilemia? We know so little of her.
4. Zoya is lesbian? Good idea but we didn't see any of that (:
5. Are Pontius' knights ok?
6. We see Gislan, Carabel and a few other students in the damn clockwork palace. How do they return to the ground?
7. What the hell happens to Sunny and Goderic when they fly off a cliff (I hope they died) but I guess the Trine developers didn't say that to make it 'more kids friendly'.
8. How does the freaking academy get rebuilt? Who dies? Who lives?
9. How do the students graduate?
I hope the Trine developers will coment on this.