I ended up abandoning the project. I will have to leave the objects as-is in the video. It is a trailer for a position piece on the "war on drugs," with the resulting violent consequences, including some gruesome shots of drug dealers and suspects shot and bleeding to death. I wanted to pixelate some objects in those images. I spent three days solid with my wife helping, just to try and blur about seven seconds of footage. I fully understand the concept adobe has thrust upon its customers in doing special effects: the first video layer is clean, second duplicated layer is all pixelated, third "matte track" layer metaphorically "punches a hole" into the second video layer, and it drops onto the first clean layer, taking the mosaic "hole" with it, and applying it to the clean video. All well and good. And I took a tutorial video from Colin somebody (No Stupid Questions series), and faithfully executed every single step he layed out, including starting the motion clock. I saw the key frame tick appear. Then, as I moved through the video, I saw no further ticks. And in no frames that have video queued, (including the export dialog) did I see any mosaic. It's as if nothing I did "took." I think he left a step out, or his older version works, and my CC version has a defect.
What makes me angry is that Adobe makes you go through all the maddening little steps to accomplish something which should be handled behind the scenes. It shouldn't take more than a few simple steps to put a blur on a part of a frame. All this layering, key framing, matte tracking, position clocking etc., may be fine for techies who want to get into esoteric editing, but for simple effects, there should be some proper tools for these tasks.
I may resort later to some editing freeware, but I can't see myself paying money only to have to do Adobe's work for them because their programming staff is to lazy to provide some intelligent tools for people who don't have the time to invest months of learning curve to learn their arcane approach to ergonomic software. And please, I understand the reason for all the layering etc. for the creative process. It just doesn't work for me. It's just too much work for me.
It is a shame you gave up. We could have easily walked you through it if you asked for help soon enough.
It really isn't that hard. As you said, the clean copy, the pixelated copy and the track matte.
Heck, you can use a garbage matte on the copy and avoid the track matte altogether. Just use the 8 point matte if you need more than a four point matte. Mosaic applied hides the fact that the matte does not have soft edges. If you need a second blur, copy the second track to the third track and change the location of the garbage matte.