I agree that the finger touching shouldn't count as a downed opponent but leaving it to the referee's discretion is a bad idea. They won't be consistent and fighters won't know what is or isn't allowed. Force the fighter to defend himself and take away the finger tip rule. Make it full palm or lower on the arm (ie: rested on elbows), something exact so there's no "discretion" involved.
I also believe head strikes should be allowed if the fighter delivering the strikes is on his back, ie: upkicks, even if his opponent has taken a knee or is otherwise "downed" as well.