Loading, Saving and Mixing Animations on custom build rig (3ds Max)

Hi,

I got another question.
I´m currently still working on a character set up for my boss.
As I´ve mentioned before, I´m trying to convince him to let me build a custom bone rig instead of using CAT.
One thing I´m looking into right now, is how to reuse animations for a custom build rig, similar to CATs animation layer system.

A) I know you can just save and load animations via the animation menu and as long as they are for the same character its pretty straight forward.

B) I tried using the animation layers from the animation menu to be able to mix several animations together ( like for example have some head movement on a seperate layer to be added locally to another animation).

First problem with this approach: Since I´m adding the animation to a seperate layer I now have to remap everything, wich is annyoing and I also think my boss wouldn´t want to have to take care of this process on his own.
Second Problem: As soon as I enable layers the frozen transforms no longer seem to work…

C) I also took my first look at the motion mixer.
At first glance this looked like the perfect replacement for CATs layer system.
But then I realized I couldn´t keyframe my character manually anymore, I could only use saved clips…

So. Any ideas on a suitable workflow, anything I´m missing?