Excessive artificial D2 supplementation can be dangerous but they have stopped making it for that reason.
As for you asking about vitamin D2 absorption. Vitamin D3 is far more bioavailability to humans, where as vitamin D2 quickly leaves the body and vitamin D2 can't convert as easily to the vitamin D hormone 1,25-dihydroxyvitamin D(3) (1,25D(3)) which is the form our bodies actually use.
But like I said for vegans and vegetarians it's the best option they have other than sunlight and over the counter pills.
It sounds like you understand this stuff pretty well! You might enjoy reading the sources they cover this is much more detail :)
Of course don't forget to eat your pears for their boron which'll enhance 25-dihydroxyvitamin d3 levels and prevent its clearence to 24-dihydroxyvitamin d3.
Thank you for the insight friend :) Followed!
For upvotes and commissions I'll answer any question you may have kind sir. I'd be more inclined to post such insights as well in the future. I just added a novel to your eastern medicine post. Few insights included though. Perhaps tomorrow if I'm feeling particularly motivated