Alignment Drift

[Callista]

Alignment drift is normal. You should expect it to happen. For most characters who experience it, it goes along with character development. It's not a sign of being "unstable" at all.

It can also happen early in the adventure while they are still getting to know their characters. If you have a problem with this, the easy solution is to leave Alignment blank until the second or third session, at which point they should know which alignment their character is.

I don't see how alignment changes "foil" the DM. If you're thrown off by alignment changes, just wait until the PCs decide to unseat the King, start a world-famous tavern chain, or join the vampire BBEG you expected them to fight! DMs have to be flexible. If you can't do it on the fly, there's nothing wrong with stepping out for a few minutes to figure out how your world responds to the curveball they just threw you.

I don't think your issue has anything to do with alignments, really; I think you simply need a houserule that everyone has to create characters that would logically work together.