Agreement I agree. I completely agree. I totally agree. I couldn’t agree more. I can’t agree more. Disagreement I disagree. I completely disagree. I can’t agree. I don’t think either of your expressions is necessary but “I can’t agree more” has the risk of being confused with its opposite, so it would be my least favorite. But you might hear it. Personally I would stick with agreeing or disagreeing with a modifier for emphasis.

Could refers to the future but Can refers to the present. It is a very slight difference. so in the spoken language either might be counted true.

I've heard both, but I agree with Simon that "couldn't" sounds more correct. I don't think there's any difference in meaning. If you want to use the phrase yourself, I recommend sticking with "couldn't."

"I couldn't agree with you more" emphasising agreement is widely used and correct. I expect you hear "I can't agree with you more" sometimes but is sounds wrong to me.

Thank you for your answer, Erin.