Assuming their “reference client” is actually true Vanilla (Patch 1.12.1). Then they should be able to drop in the original content since it wasn’t until Cata that they changed from MPQ to CASC. The biggest issue would be their apparent pathing system change that happened like right as Naxx dropped.
But…I mean, c’mon. They had time to give Retail the “OG” (not really still was post-1.8 map) AV.
They definitely would have had to redo the server code handling AV objects (ie NPCs and their pathing). But it’s not like they hadn’t had ample warning NOT to implement “1.12 AV”.
So…like the time they spent making the anniversary map that was used for, what, a week? Would have been better spent giving Classic players the version they wanted.
Right now, they should just enable XP in AV (it’s still the Legion Client and Server, just with modifications to make it look like Vanilla). Because right now there’s zero reason to go into AV. Even as Horde, you’d be better off getting a premade team together and rolling Alliance in AB.
At least with XP, there’d be some life breathed back into AV. And maybe, just maybe, Alliance would actually stay in the towers till they cap. 
((Note: When I say “XP” I’m talking like the 2k per tower you got; scaled obviously down to match the 52-59 xp curve…probably like 500 per tower destroyed, and like 1k for victory. Basically if you play and lose every game, it’ll take about 10 matches to get a full level.))