Global volume controls, via the Resource.xml, do not seem to be supported on MC for Mac. I think they are supposed to work on windows, but I don't know for sure.
As for the media keys on your Logitech keyboard:
1. MC is supposed to obey these if the media key mode is set. Take a look at:
tools > options > general > advanced > media key mode
I don't like media keys and I only have them on a couple of laptops which I don't use. So I don't have much experience to guide you.
2. It's odd that Play/Pause launches itunes. Except that the Mac really likes itunes and lots of things seem to interface with it. I just did 3 minutes of research and found that there are several workarounds for this, but none are particularly elegant. I recommend you do a search on something like "mac stop play key from opening itunes" and read some of the articles you find. You might want to do the disable that is recommended in several of them. I have never tried any of this, so proceed with your own judgement.
Good luck.
Brian.