Hello! For your questions:
1, Yes, Scavenge is the most well-rounded, viable doctrine that's almost a must-have in your loadout. It gives you good, infiltration sniper units, Ostwind (OKW doesn't have it by default!, it CAN help you overcome the disadvantage after light vehicles rolled out. It's good against infantry, planes and the light vehicles that will be on the field by the time you get it. Another ability, probably hands-down the best cost-effective arty in the game (after the 7.5 cm ISG support ability from OST mech assault commander). An improved salvage that gives you +5 fuel (standard salvage) and +15 munitions (that's the upgrade). It also gives you a nice grenade assault that's good for garrison cleaning (if you want to knock down a wooden house from full HP, it's your best bet) as it costs 15 munitions and every alive member in the squad throws one.
2, The main disadvantages for OKW is probably the lack of handheld AT (yes, sturmpios can have 1! schreck, but they're costly so you don't want to get mb bleed). I think 1 SP with sweeper and 1 with a schreck can be a solid combo. Sadly allied early game can be so aggressive (yes, I'm looking at you penals and M3 HT). The other disadvantage is the lack of early snare so a lot of people like to use T1, flamer penal + M3 against them as soviets as you can't faust it till you convert a truck, so you either get a raketen early (270 mp, volks cost 250) or you try to survive it. From USF, mech company can be hard in the early game as it gives them the WC 51 jeep with .50 cal on it, and that too can garrison a riflemen squad. Last disadvantage is that you can't trade in fights like soviets as the units are quite expensive.
3, I'm not sure what to say in advantages. It's not an A-move faction any more, but if you go mech and puma you can easily deal with enemy light vehicles. Another one is the Kübelwagen as it can outcap the enemy and doesn't bleed you precious manpower.(mostly in 1v1 tho, but can work in 2v2)
4, Yes, one mistake and you are done
Just kidding, but the worst mistake you can do is losing squads. Not just early game, but the whole game. OKW has 5 levels of vet and it's really painful if you lose a vet5 unit during the game.
5, Against tank destroyers you either need infantry AT (like raketenwerfer) or a superior TD (like JP4 with it's vet1 alien camo). Remember that fireflies cost 440/155 and stugs only cost 280/90. Tiger is an AI (anti-infantry) tank that can withstand fair amount of enemy fire. Panther is a said TD as it's only good against other armor and have great speed with good armor so it CAN act like a cannon fodder for enemy fire or even flank the enemy.
6, For 1v1 I would say that Scavenge, Spec Ops are a must, the last one is up to your liking (Elite Armored with it's Sturmtiger can work too but it's really situation dependent).
For 2v2 with OST mate: Scavenge AND/OR Spec Ops with either Fortifications or anything. It really doesn't matter as you'll pick the first 2 about 60-80% of the time.
Hope I helped!
Cheers