Where are you getting the 2x going rate number? I see in this link (from your wiki page) that they are estimating the background occurence to be between 2-16 per 1M. It also explains why they think Germany's numbers are higher. The UK numbers would still be in line with the background range I found earlier (2-5 per 1M). Either way, I think it's still consistent with the idea that events with low background rates are easier to catch because each event carries more significance and is less likely to face bias from the doctor to write it off as in related.

https://www.bbc.com/news/health-56594189

"No doubt that underreporting is a real issue, but how is it different to Covid or any other drug/disease? Wouldn't you agree that underreporting is lowest in a pandemic?"

I don't think it's significantly different. The emergency use order did carry additional reporting requirements, but it also opened it up to be administered by people who would not normally administer vaccines and would not likely be familiar with VAERS, and in some cases patients doesn't know who administered it to contact them to file the report. If you couple this with the massive increase in vaccines administered, I think the number of unreported events are higher now, but the rate is likely similar to before. That's just my guess based on the reasons above and my own experiences. Why do you think it would be lowest now?

"How likely do you think your theory is? Have you spoken to a medical professional or an immunologist about your theory?"

I'm not sure how likely it is. I feel like it depends mostly on the individual, like most autoimmune diseases do. I did discuss this with a doctor and they said that it is a reasonable question and theory that appears not to have any studies looking into either side of it. It's possible we won't know for years or decades, especially since we don't even know how autoimmune antibodies levels lead to disease in general.