Yes and no. Having high APM certainly doesn't make you the best gamer ever. However, training your APM can train your micromanagement abilities and might improve your reaction skill.
The score is a number made by analyzing your session data. It was made for fun, so don't take it too seriously! To make it even more fun, I'm keeping the formula a secret.
There is no "maximum" score, it simply gets higher the "better" your APM is. Note that the formula might change in the future as well.
I don't consider myself a pro so I'm probably not the right person to give advice on this. I'd suggest looking it up in your favorite search engine.
There are no plans on making a Linux-version at the moment, mainly because I don't use Linux that much myself. I won't say never though, if Steam OS gets more popular for example, Linux might become an interesting platform to develop for.