Once you've got this new cert. you can MITM, but you can't use it to decrypt the traffic already captured. Also anyone paying attention sees the cert. fingerprint change out of the blue.
A) Law enforcement doesn't need to decrypt previously-captured traffic; they either want to fish for criminal activity or they'll allow their target to build up new incriminating evidence. B) Who pays attention?
A) That's what they were after though: “all information necessary to decrypt data stored in or otherwise associated with [the account].” A rogue cert and MITM would get the password for the account though, unless B.
B) Anyone who knows what they're doing and has something they really want to keep secret? Maybe if someone had such a secret they'd learn to check the cert, maybe even install an extension that would highlight unexpected changes.
Once you've got this new cert. you can MITM, but you can't use it to decrypt the traffic already captured. Also anyone paying attention sees the cert. fingerprint change out of the blue.