Two questions about masks

Hi all,

I have a semi-functional relationship with FCPX here - I'm a musician who uses it to make quick and dirty music videos. I've no actual desire to get serious about my video skills for their own sake, only in the service of my music. But I also don't have budget to hire this stuff out, so here we are. I've made several vids in the past and I'm always looking for some schtick to get eyeballs on the vid, and therefore, eardrums on the music.

Anyway, I have two questions about masks, related to the video I'm currently working on. Here's the context: the video is a stack of old TVs and oscilloscopes on which I'm displaying the audio from the music as an oscilloscope waveform, and the band members performing the song on the various TVs. This is all being done with clips of the oscilloscope waveform, and clips of the musicians performing. I'm adding masks on the screens of the oscilloscope and TVs in the main vid/clip, and allowing the aforementioned performance clips beneath to show through after moving them to the appropriate places. Here's the idea (this is a private youtube vid, so pls don't share it obv :-) : https://www.youtube.com/watch?v=PckY3vEfU_g

1. Is there a way to change the opacity of the maks, or fade them in/out with keyframes? I'd like some of the screens to stay gray until they're in use, but it looks jarring to have them immediately turn to black. The black mask also looks weird when nothing is being dispayed behind it. (The background of the various clips doesn't have to be black, that's just what the scene removal tool leaves it as.)
2. I discovered the "tracking" feature when I was drawing some of the masks this afternoon. The current main clip is static and shot on a tripod, so I can leave the masks static. However, I do have some handheld clips of the scope and TVs too, and it might be cool to make it look like they were actually playing the various shots. Can I use tracking on the mask to follow the scope and/or TVs in a handheld shot and keep the mask locked to that scope or TV screen? if so, is there a primer on how to do that? Secondly, if that works, can I pin the performance clip to the center of the mask and have it follow in the same way? (hope that makes sense)

Here's the basic idea for the vid, hopefully this will give a little more context:

I've done some "projection" in the past with pixel films auto tracker, for a related kind of effect. It worked out well in the end, but getting the timing of the performance right using their drop zone paradigm was a giant pain in the arse. here's how that one ended up: https://www.youtube.com/watch?v=s8qCLNsWM2A

I don't think the PFS auto tracker will work for this though, as I'm displaying the performance clips "behind" the main clip if that makes sense.

Thanks all, happy Friday!

Geoff