420 is in the low range. The new cut off is 400pg/ml in US.
Other countries may have different measurement concentrations and would have to be mathematically adjusted to US units.

Your swelling may be only internal. GI and breathing. I didn't get
the skin swelling until my angioedema was SEVERE. Most of mine is GI.

If you have the genetic version, you don't need a drug trigger.

Start reading my B12 thread. I strongly suggest you start oral
supplements, daily. 5mg a day on an empty stomach for 3 months and then get retested, to see if that level is better.
If your neuro symptoms are due to the low level, they should improve slowly as your new level rises.

I'd also get that ANA retested again. ANA can fluctuate (between normal and abnormal), and lupus is a major cause of nerve damage/pain in some people.