Replace a MaxProp with a fixed prop? Oh the humanity !!
Regarding question 1: Yes, no question the MaxProp is better for moving the boat in reverse than a fixed prop (less prop walk, better acceleration from a standing start).
Regarding question 2: The additional drag may not be a big deal for local short sails, but it'll add up to be significant for long distance sailing. Other previous threads here at Sailnet have discussed this point, so you may want to search for them.
Regarding question 3: I'm not familiar with the Campbell, but I'd be suspicious of a vendor's claim of greater efficiency as the science of prop design is well established. It's important to match the prop's performance with your engine to ensure it can reach its specified maximum RPM (else you risk shortening the life of your engine).
If there's a chance of refurbishing the MaxProp, possibly by sending it to PYI even (call them and ask them about it - it may be less expensive than you think), I'd consider doing that. If that wasn't feasible, I'd look at another MaxProp or perhaps an AutoProp.