Once you've announced that you're casting Klauth's Will, players can't take any actions until you've finished doing so. Notably, opponents can't try to remove your commander to change how many modes you may choose.

The commander you control doesn't have to be your commander.

Whether or not you control a commander is determined only once, as you choose the modes for this spell. If you somehow lose control of that commander before you finish casting the spell (perhaps because you sacrifice it to activate a mana ability), it won't change the number of modes chosen.