Personally I think it is important to have the blood testing done first before you start taking things. I can see why a lot of those things help most people. I had a short LP after our dd was stillborn. It was about 8-9 days for the first few months. I started taking 100mg of B6 and my LP went from 9 days to 12 days long. Its just a vitamin so this is something you could take anyways, but be careful of taking too much because there is probably some in your prenatal too. Oh and B6 is also great for morning sickness so its healthy to take during pregnancy as well. The baby aspirin is great and I have been taking that since my loss, but I found out that I have clotting disorders, so I also would need blood thinners during pregnancy. This is the one that I think should be taken only if dr. finds a problem. If you start taking it before the testing then it will alter the results of the test. And progesterone, if you didn't need it in your first pregnancy then you "probably" don't need it again, but you should get your progesterone levels tested after Ovulation to see if you actually need it to sustain a pregnancy. Some of the clotting disorders only show up during pregnancy, so it would be ok to go and get the test done now while you still have the hormone in your system. After a stillbirth those clotting panels are usually offered.
I am so sorry that you are going through this. Big