I can see that working on something as flexible as a collar where you can really get the collar rolled under the four fingers.I picked up the other day that especially for collar chokes it works better if you relax the thumb
That Mercy game is just plain nasty.I have found that the Mercy Game was a great start for building strength and endurance.
Also work on squeezing your fingers together like theirs are not there. the squeeze causes a compression and then you can work the rotation for the win .
I like the squeeze as it addresses tendon strength in a direction not normally addressed.
Also as mentioned Cane work. Strike a heavier cane into a heavy bag will help build grip strength.
It will also train you to relax your grip and tighten for the strike. A constant tight grip can cause cramps in the forearms.