If you have them on tape saying you're assigned a CD, it's a CD. And if your crew is on a CD, that's all the ammo you should need.

I've been assigned an overnight when the rest of the crew was on a CD, but I never had them on tape saying I was covering a CD. Dunno if it's even legal, but a union rep said he thought it probably was. But see what you can do with it.

A reserve is released to rest for the remainder of the day upon completing a CD. So no, you are not on duty after returning to domicile tomorrow (Monday).

If they give you something tomorrow, ask for a CP. Have a CP listen to the tape and if they insist you take additional flying, do what you must and grieve it. Should be good for an extra day off.