"I couldn't agree more (if I were to give my opinion)" is in the subjunctive mood.
"I can't agree more (if you or anyone asks for my opinion)".
These would be nearly synonymous expressions with only a minor grammatical difference in tense, which has little bearing on the sense being conveyed.
"I agree", however, without any modifier could be anything from full agreement in every aspect of the social context as well as a reluctant choice with unspoken reservations, a concenssion, or limitations. Hence I would not say you could used the previous two and "I agree" interchageably in all situations. Use them interchageably only when you fully agree, and you know for certain what far-reaching impact their/your statement might have.
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