If you begin a charge, but half-way through you feel with tremorsense that you're running into an ambush, can you interrupt the charge? If so, I assume that you'd lose the rest of your turn because you still used a full-round action.
Your GM shouldn't put you in that position but I think you can and you assume correctly.
If you begin a charge, but half-way through you feel with tremorsense that you're running into an ambush, can you interrupt the charge?
Yes. The charge can be terminated voluntarily (or involuntarily) but the penalty for charging remains until the start of your next turn as usual.
If so, I assume that you'd lose the rest of your turn because you still used a full-round action.
Yes.