Re: the mark left - I get that sometimes too but it fits in with my patter.
The version I do goes like this.
I ask for the Queen on the 1p to be made to look hard - give her a goatie, an earring, long hair etc. and get them to sign the tales side.
The 2p I ask for them to sign the heads side and on the tales, draw themselves as a stick figure, also looking menacing.
Both coins in hand - "Name a coin"
FIGHT - they jerk their hands from side to side "You want to knock your opponent into oblivion!" (I make it sound like the coin's going to disappear.) This goes on for a little bit.
"One last punch - BAM." They stop "Now shake your fists, are there still two coins in there?"
"Yes"
"Oh.. (fake that the effect has failed)... Open your hands and see who fared best?"
"WOAH!!!!"
Result.
I found with the coins jumping around in the hands it covers for the mark if anyone questions it. After all "it must have something to do with the warmth of the hands" was once used to explain the bend.