1) The OptiMate is compatible with all type of lead acid starter battery. Please note that the ETX batteries (BMW OE fit) are an Absorbed Glass Mat (AGM) and not a gel electrolyte. These are the most common type of powersport batteries available and partly the reason why the OptiMate was developed. There has never been a reported incident of an OptiMate causing a battery defect and with over 1'000'000 have been produced.
2) If you charger is showing test results (Green Good or Red Suspect) then the charger will be functioning correctly. So there should be no need to replace it.
3) There have been quite a few changes over the years (even within the OptiMate III's lifetime), most of which are very minor improvements and the only significant changes with the introduction of the IIISP are; Increased ingress protection to IP42 making the charger Splash Proof, the end if the recharge stages are indicated by the charge LED flashing and charge current is now reduced at high ambient temperatures.