How much trouble is this worth to you...
This is worth quite a lot of trouble! If I don't get this right I may be looking at a very mangled and difficult to replace machine! 
I am not sure I understand how to implement your first answer. What kind of switch would that require and how would that be wired up?
Are you saying it should essentially work like:
SWITCH OPEN:
A = 5V
B = 0V
SWITCH CLOSED:
A = 0V
B = 5V
Then use a comparator on A and B with the output connected to the Arduino interrupt?
Your second idea is exactly what I am using with the hall sensor. I guess I could also do this with the switch... thinks