When i opened up the group to get at the brew valve i did have a look at the seat. It looked ok to me, i also cleaned out the area with a toothbrush and some vinegar.
I can't say i took a good long look at it though, it's not impossible that there is a slight dimple or other surface anomaly that is responsible for the valve having a hard time to seat, if that really is/was the cause for my low idles.
I had a similar issue once with a faucet, had to change out the seat and seals, but that was a relatively easy fix because of its cartrige design. Can't image what it entails with the e61 group, looks like you'd have to replace the whole body
209 idle still holding as of this morning. I'm also back to my usual flush and go routine. Flush the group, goes up to 214-215, stop at 210 lock-in and go, shot ends at 199-200. Keeping my fingers crossed!
On the bright side, i finally got a chance to get to know the internals of my machine i bit more. I'm sure this will be useful sometime down the road since i intend to keep this machine for a while.