It's down to the fact that you can't log the required IFR time for cross credit because you don't have a valid rating allowing you to fly single pilot IFR. You need the IRI or IMC/IRR to meet that requirement.
I did my initial IR in the DA42 then went multi-crew so much the same situation. Your ME-IR contained SE-IR privileges, even if the SE-IR was never issued as a standalone rating. (It may have appeared on your licence as IR-SPA-ME/SE class)
Yes, as you hold an SEP land you can do the 3x approaches and get the SE-IR privileges back again.
Edit: Where are you based? There are a few of us here with FI/IRI that might be able to help out!
