Notritter,
Well after deploying the project and trying it on a different computer with the new videos, the other computer did not have the black flash on transistions.
Turns out my nividia graphics installation at some point became corrupted. Got in touch with support there and got those issues fixed, and voila, no more jarring black flash transitions with your plugin!
So with this, and the correctly coverted webm files, both of my issues are fixed. Both problems were on my end. My apologies for wasting your time, and thank you again for your reply and work on the plugin.
Have a great rest of your week,
-Niniann
PS: Your information that you did not have these issues on your end were key to figuring out that the problems were with my own methods. So, thank you for this information :).