I second the Goofoff or GooGone for removing the sticky residue. I've never had it damage any bow finish, but you should wipe it down anyway with some mild, soapy water to get rid of any trace adhesive and the petroleum-base cleaner.
I think you will find the brace height has quite an effect on wrist slap - at least it does for me. Get an arm guard anyway. You will need it to prevent the string catching on a jacket sleeve in the colder weather. I generally don't bother with one in the summer unless I'm shooting a low-braced longbow, but definitely need one in the winter to hold down my jacket sleeve.
If that string continues to be a problem after moving the nocking point, a good string is one of the cheapest improvements you can make to the bow.
Enjoy your Montana. I have never owned one, but had an AuSable. Very close except for a bit of r/d on the AuSable. They are nice bows.