I didn't believe I had OI, either, and I was SO wrong. Treatment has improved my quality of life. I can do quite a bit more than I could pre-OI treatment, but I still have PEM. OI treatment did not cure PEM/PENE. It did, however, increase how much a can do before PEM hits, so there might be some connection.
You should probably be tested for delayed POTS. The short test won't do, especially if your symptoms are not obvious. You should be standing (or tilted in a TTT) for at least 30 minutes in order to catch the delayed form.
Some of your symptoms could indeed be OI-related, but I don't think swollen lymph node would be. You should have those lymph nodes checked out by your doc, if you haven't already. They could be ME/CFS-related, but they could be something else entirely and should not be ignored.