I am TTC number two.
Can't answer all your questions... but I also have had rather low temps and I remember from my reading that it's important to have your thyroid tested because that can sometimes be the culprit for low temps.
You might also ask your Dr about doing BW at CD3 (I think that's when it's usually done for estrogen, thyroid and stuff....)and then 7DPO to test your progesterone levels.
Another just practical thing is to be sure you're not sleeping with your mouth open, which.. blush.. I'm sometimes guilty of. That can cause your temps to dip. :D
Also, I took Vitamin B6 (100mg) and it lengthened my LP to 15 days