If I'm waiting for the mac version can I still activate my key and tie it to my steam account or is this key PC-only? I guess what I'm asking is will this code work when the beta becomes cross-platform? 

This code should work cross-platform, but I think if you activate the Windows build it will lock you into that build permanently. If you are worried about that, I would suggest holding off on activating the game until we release the Mac version in a few weeks.

Oh okay, I've made an error then. I guess I thought I would be able to start playing it on my PC and then later play on my Mac (which is newer/better) when it became available as long as it was tied to the same Steam account. I haven't downloaded the beta (or anything) in PC mode but I did generate a key and link it to my Steam account (I see it in the library but it's marked as 'not available on your platform') - do I need to contact support directly or should I be okay later? 

Oh okay, I've made an error then. I guess I thought I would be able to start playing it on my PC and then later play on my Mac (which is newer/better) when it became available as long as it was tied to the same Steam account. I haven't downloaded the beta (or anything) in PC mode but I did generate a key and link it to my Steam account (I see it in the library but it's marked as 'not available on your platform') - do I need to contact support directly or should I be okay later? 

This should more than likely be fine since it sounds like it associated properly. If you are unable to download the Mac build when we release the game on that platform just send a support ticket and we will get you a brand new key.

Excellent. Thank you kindly. I sent an email to support because I panicked. Please kindly ignore it.