Wow there is a lot going on and i dont know what to comment on first lol So ill just leave it as this for now.
1. Please post your MRI results when you get it. I dont expect it to be an occlusive vascular event as your calcium score was 0 ( mine wasnt even that low lol) and your 5 year risk for an event is almost nothing. However without more information on your type of MRI ill assume its without contrast brain and that will show is the general patency of your vessels even without doing flow specific sequences.
2. Probably jumping the gun a lot when it comes to seizures. Most people your age IF they had seizures it would be from a mass, mesial temporal sclerosis, or congenital but if its just presenting now im gonna move away from congenital although not impossible. We can see almost all that stuff really well on MRI. Yea theres a million things on a differential but its pointless to list them.
3. Your concerned for autoimmune. Thats fine and its on the differential but lets say its something like MS we have a really good chance of seeing that on the MRI. If your concerned in general you can easily have your physician do a general test like ESR or CRP. Not specific but decently sensitive. With the limited history you have and symptoms I would not start ordering every test.
4. Someone mentioned GCA. I like it. Still a little young but not impossible. The ESR and CRP will help with that.
5. Your legs. I dunno. If on exertion but your calcium score was low im less likely to think occlusive. Could be either nervous system related but if its the same leg all the time it may also be an entrapment like maybe your peroneal getting squished by medial head of gastroc or something. Not sure. Or you need to get in better shape
Ill check back in after you post up the MRI results as we are all just guessing without more info. When you see your PMD they shouldnt have an issue adding the ESR and CRP. Its good to get done once in a while for fun.
Please note: What i posted above is NOT OFFICIAL MEDICAL ADVICE. Im just rambling on while eating some dinner and replying to emails. Its a general discussion and not complete in any way. Yes i left things out and yes I made generalizations.