(Disclaimer: I don't play around with the market gardener that much, if at all, so I might get this wrong entirely. What I'm about to say is based off my experience with grinding jump maps and some hasty googling. If someone is more versed in this please help clarifying.)
What bhopping is in a rocket jumping context is bouncehopping, not bunnyhopping. (iirc the reasons why they can be made in the first place are the same, but since I really don't touch bunnyhopping that often at all so again, I may be wrong.)
A bouncehop is where you hit jump (jump only, no other inputs until you make the hop) as you touch the "ground" (not the actual ground, like 1 to 6 hu above it). Being airborne in tf2 means not touching the actual floor, so if you were to jump on these psuedo-ground you can still be counted as airborne from the previous explosive jump, because funky source engine quirk. Therefore the crit is maintained.
The thing about bouncehops is that while you can do them after you fall from any height, they can be really inconsistent because of the window of time you have to press jump. There is a list of set heights that is consistent if you fall from them and jump maps have been incorporating them ever since bounces were discovered to be a thing in 2013 but that's about it. It's really about learning the timing to them.
Personally I would recommend not binding mouse wheel to +jump because it's better to learn the timing than to rely on binds, not to mention in recent updates they made it so you can't spam too many commands at once, otherwise you get kicked from servers. Then again, it's personal preferences so just try until you can get it right, it may take a long time but that's really how it goes with most things in jumping.