I've been through the issues you have faced but did get reward space in ET by calling. With the exception of the 814 every day there is currently I5 availability on nearly all the flights (19/20/21) and so you are pretty likely to get the MMB proactive upgrade after the ET purchase if you really want to go CE.

For a vacation and to get where you want to be, when you want to be there, I'd nail the F availability, buy the ET seats and not sweat it any more! YMMV

Oh and if you have a longer layover than usual as you can't get the shortest connection a cabana in the CCR is a very pleasant base to help while away a few hours.