How common is it to train for one MOS, but then quickly transition to another after IET and joining your NG unit?

Don’t sign anything unless it’s what you want if you want 11B don’t be an idiot and sign a 12B contract just be patient and tell your recruiter you want 11B and that’s final

The only thing guaranteed will be what is in your contract.

The availability of an MOS in a National Guard Unit depends on what units are in the state and from their website they have slots for 11B's and 12B's. I suspect that you have a very good ASVAB score and that the recruiter is trying to fill a slot since the line scores required for a 12T are higher than for an 11B or 12B, which have the same ASVAB line score requirement BTW. It has also been my experience that National Guard recruiters try to fill the vacancies from the local units first, so if you don't have a local infantry unit, they won't push that MOS. I have a very good GT score and the list of MOS's the recruiter offered did not include 11B, which is what I volunteered for anyway and again when I commissioned. So if you want 11B, tell the recruiter you will just wait until a slot opens and my guess is that he will find an open slot.
OTOH, I branch transferred to the Engineers when I moved to Kentucky and I liked duty as an Engineer. If you are looking at the Army as a way to get some real world job experience, the 12T translates back to the civilian world a lot better than an 11B. I was lucky and served in a Divisional Engineer Unit, which supports maneuver units. Translated that means that I got to be an engineer and still play with the tanks and blow stuff up.