Is there any point to still try in undergrad other than having Honors on my degree?
Could I just get all C's and it not make a difference for my future?
You will have to spend them your transcripts before you enroll, although they won't dig you for that. Some employers may ask for UG transcripts if you went right from UG to law school. General advice, keep your grades up. Besides you don't want to pick up any bad habits right before LS. Remeber that poo goes on your perment record.
Can they accept you and then deny you based on your grades Senior year?
Just read the fine print. Most schools once they offer you acceptance, and you accept (including putting down the seat deposit) then it is irrevocable. Some really elite school, however, may have something in there that says they can reject you if you bomb your last semester.
As for firms- they really won't ask about UG, but it will help you to be able to say you did well in UG when they tell you they are looking for a "hard working, motivated, and extremely intelligent individual."