I exactly did the same thing this week in my current project. We had to move the server and QV both from 32 Bit to 64 Bit and I did not had any issues at all, even with the Drivers.. I think there will be an issue if you started with 64 Bit and trying to move on to 32 but other wise its just fine and I am happy to say this.. Please try it and let us all know if there are any issues.
On #1, I believe the isnull() function behaves differently in 64 bit QlikView. I always use the len() function instead. If len(MyField)=0, then the field is null. That's all I'm thinking of. We develop on 32 bit machines and publish on a 64 bit server, and have had no problems. We don't use Oracle, though.