Can I choose to not remove all action tokens?

In HeroClix, may = may, can = must.

So looks to me that you don't have do do anything after the may. Johnny can stay.

There's only two things you can do.

A. You remove all tokens and replace him with ASM Ghost Rider on his Orange Starting Line.
B. You don't remove any tokens and don't replace him.

May gives you a choice, either you do A or B. You can't not remove any actions tokens and replace him, the "and" wording in his Trait makes sure that you do both actions, not just one.
Trying to activate his Hellfire Chain Ability eh?

YEP!

Thanks.

I would concur that removing tokens and replacing him go hand-in-hand. Otherwise, it would say "[…] if you do, you may replace him […]" instead.