if you have mild endo then extra long down reg plus 5 day letrozole makes perfect sense to me, but I can't remember if it was you or another lady who had lining problems on letrozole before?
no sense in throwing away a chance to try to get pg naturally in the meantime!
aching straight after ET is too early to be immunes, sore throat in 2ww might be.
remember that DE success rate for fertile ladies with no immune probs is about 60%, so there may have been nothing wrong, just that this time you were in the 40% - for some of us, even though there is no huge issue it still takes 2 or 3 goes with DE to get there.
what about the sperm? just because it tests ok, you can never be sure its not the problem. if you had 2 or 3 goes with DE and it still didn't work, you'd have to start suspecting a hidden prob with sperm.
yes, I'd do LIT - on the grounds that it can't do any harm - and there is a (small) chance it may help even if you aren't an RPL case.
I did try cordy/reishi/coriolus combo and I did get pg, but I can never know whether it was a signficant factor or not.