Dave LaRonde has been around a long time and I have been reading his posts for years now. If you have had any experience in updating Adobe applications then you would know there is a laundry list of things that could go wrong. It NEVER goes smoothly. Adobe is even aware of this issue and has created a way for you to install 2015.3 separately instead of upgrading 2015.2. Now you can run the programs separately instead of having to uninstall and revert when these issues occur.
Just a note, the animation codec can bog down a computer pretty easily(even if it is a fast one). If you can play it fine in quicktime it might not be a problem but if it lags in quicktime, I would look at another codec to track in like a ProRes 4444 depending on the quality of your footage. Mochas tracking is better and easy to use. After you track in mocha, it copies the tracking data to your clip board and it is as simple as copy and paste.
Add 4k vid to render queue click “output Module”(where it usually says lossless), tick the resize and plugin your resolution. 50MBS should be just fine. Not sure how you are calculating your downscaling but here is a resolution calculator – https://bneijt.nl/pr/resolution-scale-calculator/.
Toggle track targeting. Meaning, wherever your multicam sequence lies, target that track by clicking on V1, V2, etc., for multicam monitoring. This should not be confused with source track targeting.
Well just to test things out I did try and slow down the credits I exported from AE at 60fps to about 30% speed in FCP. It looked much worse. Maybe there is another program other than FCP or AE that specializes in making credits?