weird preflop situation 50nl

[rpm]

BU opener is 26/20 with a 44% ATS, 5% 3bet, 85% fold to 3b, and 0% 4bet. over ~150 hands.

BB is spastic fish who is tilted and pissed off at me. earlier in the session i fired two barrels with an OESD and folded to his river minraise. since that hand he has been 3betting my opens and raising my cbets at almost every opportunity he gets. i also correctly check/called him down 3 barrel bluffing when i had AK high which backed into a pair on the river. PT3 has him as 39/18 with an 11% 3bet over 50 hands or so.

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) - PokerStars Converter Tool from FlopTurnRiver.com
BB ($76)
UTG ($88.20)
MP ($63.80)
CO ($56.20)
Button ($50)
Hero (SB) ($81.45)
Preflop: Hero is SB with Q, Q
3 folds, Button bets $1.50, Hero calls $1.25, BB raises to $5.50, Button calls $4, Hero raises to $17.50, BB raises to $34, Button calls $28.50, Hero?

(BU has $16 left in stack, fish has ~$42)

i flat the open because the BU opens pretty wide, folds to 3bets a ton, and doesn't seem capable of 4b bluffing. plus i expect the fish to pop it a huge % of the time when he thinks i'm weak. once he does this i decide to 4b to give him some rope. i make it pretty large because we are ~150 deep and he probably has no idea what a standard 4b size is. the fish snap 5bets it (which is what i expected him to do a ton) and the reg flats.

i'm obviously stoked that the fish 5bet, and am more than happy to get it in versus him, but is the BU's 5b flatting range too strong to even continue? if so, does that make 4betting a mistake?

is flatting the BU open OK given reads/dynamic?
i assume 4betting to give the fish rope is pretty standard? sizing?
preferred reaction to fish's 5bet AND BU flat?

i get the feeling my question still isn't being expressed clearly enough but oh well. it's far better than it was.