When wiping the head in wudhu, if a person accidentally wipes too far and one or more of his fingertips touch his forehead and thus the water of his fingertips mixes with the water of his forehead, does this make the wudhu invalid and in turn does he have to start his wudhu again or is it permissible to continue and wipe his feet?

That does not invalidate the wudhu. The face has already been washed and a part of the head just needs to be wiped. There is no issue with that.