They can be quite pretty eh ?
No probs with the DMT C for lapping, of course as long as you're sure of it's flatness & no probs submerging the Asagi. They are very dense hard stones & will not suffer at all for the time it will take to lap. Of course if you had the Atoma you wouldn't have to worry about the plate sticking to the stone as you lap
I would not use a synthetic stone for slurry. Just use the DMTC for now. It will work just fine.
Enjoy !