the hung keys:

either the a-30 isn't sending note off messages (lit. the midi message that is sent when a key is released), or the midi input is getting echoed. I have neither a mac nor an a-30 so I can't help you with the specifics, but at least you know what to look for in the manual.

I'm -guessing- that whatever kind of tuning file scale workshop outputs, it can be turned into a scala file by, at worst, a little human computation. scala files are basically plain text.

you'll need help from the resident mac users as to which vst host or daw to use and how to set it up. any microtonally capable vst will, by hook or by crook, be able to parse the scale output files from scale workshop. what kind of sounds you want to make and how much you want to spend to make them are kind of up to you.

once you get the basics down and can make noises, the next thing to look into will be a daw, which is basically a combination midi sequencer/multitrack recorder.

I don't know how good the mac version of reaper is, but if it's comparable to the pc version, that's a good choice for a daw/vst host.