I had a similar situation here. You handled it very well. I must say the letter was pretty impressive and the advice about your W being sweet one day and mad about being sweet the next is all par for the course. In my sitch, I actually confronted my W and of course she played everything down. Unfortunately (or fortunately) I didn't have the actual text of the messages but the frequency and time of the contacts (when I wasn't around) spoke volumes.
I think some of the WAW might do something like this to give them a place to jump off to, i.e. they start to prep another man to run to before they leave the first. Just my thoughts.
Presently, I think my W has pretty much dropped all contact with the EA/OM and things seem to be improving, even if we are separated.
TwinDad Me 39, W 36, M 11 W - MLC, WAW???? 2 Kids B/G 3 YRS Old Start of the Long and Bumpy Road..... On the verge of piecing.....a new beginning