iv'e litraly just came in from re-doing my grind this time using a metal cutting disc (4mm thick 115mm diameter) which came out alot better than using a grinding disc. why not try a flat headed screwdriver to pop the tyre off, just make sure you dont stab the inertube lol, not to sure about holding the tyre out the way sounds very dangerous if the grinder catches the edge of your rim and kicks off the out come could be bloody. i take the whole wheel out, remove tyre ect then put your foot on the rim but out the way to stop it moving then just grind away