Hey MrsPOP, seen you around the boards and am happy to help any way I can.
If you want to start small on the Bvits, go with a B-50 complex. It will make your pee bright yellow, but there's no harm to it, promise! You have to take massive doses to cause any harm as the excess just comes out in your pee.
Also, there is no reason to worry about taking it during the 2ww... doctors recommend it to some women to help with their morning sickness... I'm still on my B-100 and I'm 7 weeks pregnant now, nothing wrong with it!
And your body may need the continued B vits as support for your progesterone during the 2ww.
Most GPs that I've seen won't give you progesterone to take during the 2ww unless you have a history of early miscarriage. My FS wouldn't let me take them until at least 5dpo because there is some data that suggests that taking the progesterone may reduce the contractions of the fallopian tubes and make an ectopic more likely to happen... so he made me wait until 5dpo when the egg is most likely out of the tube by then.
You can try an over-the-counter progesterone cream but its not very strong, might still help though.
If you're only on your 2nd cycle since Mirena though, I'd stick to just the Bvits and see what your body does. I had a really wacky first cycle (first 2 cycles actually) off Mirena.... granted, I went on clomid after that, lol.. since it was 4 months for those 2 cycles and the 2nd one wasn't ending any time soon etc...