I'm mostly holding back asking new rules questions until I get my hands on the final rulebook(s) and final versions of the various cards. That said an awesome person called valk shared in the KS comments some SDE photos they took at a convention which were quite revealing. Amongst other things one of those photos had a revised wording for the 'Locked Doors' Explore card:

Now in most normal circumstances the way this card works is tolerably clear, but there are certain unusual cases where I'm not sure what happens and I'd be interested to hear what other people think. These cases are:

A) Where the tile never had a spawnpoint
B) Where the tile had a spawnpoint but it was destroyed before a Hero moved onto the tile*
C) Where the tile had a spawnpoint but it was moved off the tile before a Hero moved onto the tile*
D) Where the tile has multiple spawnpoints

*The Explore card for a tile is drawn, and comes into effect, when a Hero model moves onto that tile for the first time.

My current feelings in each case would be:
A) The doors lock and can only be unlocked by the party spending a dungeon key
B) The doors unlock immediately
C) The doors lock and can be unlocked either by the party spending a dungeon key or by destroying the spawnpoint that was moved off the tile
D) The doors lock and can be unlocked either by the party spending a dungeon key or by destroying all the spawnpoints on the tile.

What do you think? And are there other circumstances I haven't mentioned where this card behaves oddly?

I agree with your B,C and D answers.

"A" is more debatable. If you accept in the above answers that "spawn point is destroyed" actually just means "the tile is now void of spawn points", then the doors would just unlock immediately.