Yay! Do NOT grant him any immunity!
I take a slightly different tack on the immunity issue
You grant a very limited immunity on a very tightly defined issue. Then McCabe has to decide on that limited issue to testify or not. If he has limited immunity on that subject why shouldn’t he? In testifying he loses easily defensible ground. If he doesn’t testify after receiving limited immunity on that subject he convicts himself.
Then come back and repeat the tactic. Wash away an other piece of the ground he is standing on.
After multiple repeats McCabe has defined exactly what he has done and to what degree.
It will take time, money, and patience; but, he will end up like the King and his new clothes. When that happens even his most ardent supporters will have publicly abandon him and attend to their own defenses.
Who says any immunity deal has to cover everything?
Wait! He wanted use immunity. That means from all crimes. Back to the cell.