for 1.

instead of the currently 582M virtual hive supply, use 460M regular hive supply

I have no problem with that as long as we recalculate inflation rate to be in line with intended increase in Hive supply. I do agree that current system increases inflation when demand for Hive is low and decreases it when demand is high, adding to volatility instead of dampening it, so that proposal seems reasonable.

for 2.
It is calculated correctly. HBD is collateralized not with existing HIVE, but with new HIVE that would be issued when conversion is finalized. Or to put it differently - internally HBD is just a different form of HIVE, that's what virtual supply represents. The only questionable element of the calculation is HIVE that is on DHF balance - since we are not counting HBD on DHF balance, we should not count HIVE on its balance either. That way whole DHF budget is excluded and there is no inconsistency.

There is other reason to not do any change in that area. In practice it changes nothing until we are close to at least soft limits, which are still way out of reach, but such change requires heavy testing.

for 3.

Imagine an entity controlling 26% of the BTC supply

You make it sound like it is a bad thing. DHF is not some shady entity - it is our common budget that we all control.

Hive ecosystem has overspent on DHF projects

Heavily disagree. DHF should be financing more, f.e. whole work of @blocktrades team should be funded by DHF, which would make it rival hbdstabilizer for spot as largest item on tab (and since stabilizer returns assets back, it is not actually a cost to DHF as its daily pay would suggest - I assume you've taken that into account when calculating that 3M per year expenditure). Hive is still young project and like any startup it needs to spend (of course it has to spend reasonably, not throw away money, but that's what voting on proposals is for). Only once the basics are set and we have some leeway in funding optional things, then DHF will spend no more than its inflation share. And guess what - spending from DHF decreases its share in virtual supply, which you said yourself is too big :o)

I'm not a fan of burning tokens, especially not in a system that was not balanced around constant burning. But like you said, it is possible to make a proposal with null as receiver and let the community decide.