Many thanks for the wonderful replies.

There is no haze in the lens at all, only a few very minor scratches... A lens hood is on my list of purchases.

The tessar lens on this model does have a single coating IIRC.

Looking at the worst negatives again, I'm verging on concluding they are underdeveloped and underexposed. I'm actually starting to think the fix didn't fully complete? Maybe I miss measure my temperatures...

Underexposed is more than likely too... I've been using my iPhone as a meter. Two photos of the same subject (one take with om-2 other with rollei) are showing totally different results.