Hi, Freddd.
I've been thinking about your body's intolerance of folinic acid and of vegetable-based folates. Vegetables contain folinic acid as well as methylfolate, and lettuce, spinach, carrots and peppers contain significant amounts of folinic acid. I suspect that it is the intolerance of folinic acid that causes your body to be intolerant of vegetable-based folate, also.
Here's what I suspect accounts for this. I suspect that you have an inherited deficiency in the enzyme methenyltetrahydrofolate synthetase (MTHFS). This is the only enzyme known to catalyze a reaction with folinic acid. If you have a deficiency in this enzyme, and you consume folinic acid, it will build up in your cells. The problem with this is that folinic acid normally acts as a regulator of folate metabolism by inhibiting enzymes in this metabolism. In particular, it inhibits the serine hydroxymethyltransferase (SHMT) enzyme, which normally is the main enzyme that converts tetrahydrofolate to 5,10 methylene tetrahydrofolate, which in turn is the substrate for making methylfolate.
So, a deficiency in MTHFS will allow folinic acid to rise, and this will inhibit SHMT, which will lower 5,10 methylene tetrahydrofolate, and thus will also lower production of methylfolate, which is needed by methionine synthase in the methylation cycle.
I think this can explain why folinic acid has been so devastating to you and why its effects are so persistent, once you have ingested it. It builds up, because the enzyme that normally controls its level by converting it to 5,10 methylene tetrahydrofolate is deficient. It stays high for a long time for the same reason. When it is high, it suppresses the SHMT reaction, which lowers the natural production of methylfolate, which then inhibits methionine synthase and partially blocks the methylation cycle.
This would also explain why you have had to take such high dosages of methylfolate, especially if you have taken some folinic acid or vegetable-based folates, which contain folinic acid. The reason is that your normal production of methylfolate has been inhibited, so that you have to supply it exogenously. I suspect that in addition, your tetrahydrofolate is probably high, because it is the product of the methionine synthase reaction, and if SHMT is inhibited, that will tend to inhibit the conversion of THF to 5,10 methylene THF, so that THF would probably rise. High THF will likely exert backpressure (product inhibition) on the methionine synthase reaction, so that it is necessary to add more methylfolate to drive it at a normal rate.