The release notes of the TK1 lack the update to V2.7B2 http://developer.toradex.com/files/toradex-dev/uploads/media/Colibri/Linux/Images/Apalis_TK1_LinuxReleaseNotesV2.x.txt.
As already with the last release and as stated here the release notes for 2.7b2 are here.
Are there any critical changes between V2.7b1 and V2.7b2?
I guess that depends on your definition of critical but have a look at the release notes and/or the full git log to decide for yourself.
Why are there 2 images now and what is the difference between them?
I don’t think anything in that respect has changed and the name should really be self explanatory. Mainline is really closer to mainline but may lack certain downstream features like e.g. hardware accelerated multimedia or CUDA.