I thought I explained pretty well in the post why it can't work any other way.

Not using the spot price to calculate collateral value leads to a guaranteed death spiral the second we get unlucky to the downside. I could mint HBD out of thin air and never pay back the loan given any flash crash greater than 50%.

Also I'm not borrowing 2000 Hive.
I'm borrowing the amount of HBD I'm given, which is 419 HBD.
A 5% tax on 2000 is a 10% tax on the total loan.
We already know this isn't true because HBD is soft-pegged to $1.05 and conversions begin there.
If this was not the case then market makers would not convert until $1.10.

You also didn't do the math.
900 * 0.45 = 405 HBD
It needs to add up to 419 HBD.
Rounding to the nearest hundredth is quite insignificant given the actual difference in these numbers.

TEST

I will run a test right now to prove I'm right.
I will convert 1 Hive into HBD.
The spot price is lower than the average.
It will use the spot price.
The spot price is 42.5 cents.
I should get 0.2125 HBD minus 5%.
That's around 0.202 HBD.

This is the kind of rounding error we are looking for.
I guessed 0.202 I got 0.204.
I have to assume I'm doing this correctly until shown otherwise.

You are right in that spot price is never used during actual conversions where tokens are burned, but it absolutely is used to calculate collateral value and loan amounts given when it's lower than the average.

Again I've done the game theory on this and it can't work any other way without risk of death spiral.