I did that Cuman/Bandit camp quest earlier than it sounds like you are, but I did the Cuman disguise one a lot later. I wish I had known about the disguise one earlier though, because everything you need is right there in the camp, and you don’t even have to kill anyone to get it (well, one person, see below). You can finish both quests without needing to sneak much or even get involved in combat until the end. Minor spoiler follows…
Approach the camp from the north, at dusk is best, and you can sneak if you like as it will provide extra cover from patrols. Along the northern edge of the ruined buildings surrounding the camp is a barn-type building which has an entrance from the outside. Sneak into there, and find a bottle of poison on a barrel and a Very Easy locked chest. Pick the lock, and loot the Cuman gear inside of it. Put on the armor and helmet, and now you can walk around in both the Cuman and Bandit halves of the camps. On the Bandit side, nobody will challenge you (they’ll just make snide comments), but if you get too close to other Cumans on their side, you will be challenged. You can choose options to fool them into thinking you’re Cuman a couple of times before they catch on. You have a good sneak skill, so you can probably work your way through the Cuman side poisoning their food pots if it’s nighttime and you’re careful. I recommend doing pots first (you may have to knock out some Cumans) in both camps, then arrows, because the second you light the first barrel of arrows on fire everyone comes after you.
To be honest though, you don’t need to poison a single pot or burn any arrows. All you need to do is scout the camp effectively, then report back. The battle that ensues will be tougher on Sir Radzig’s men if you didn’t sabotage anything, but they will still win in the end. When the battle begins, you can snipe from the edges or try stabbing a few isolated Cumans and Bandits, but if you try to stand in the front line with the soldiers, you’re likely to get slaughtered. The only required combat in the sequence is the fight with Runt at the end. Once the battle has ended, there is a chest in the Cuman tent near that barn from before, close to the food pot, which contains a crapton of Cuman gear. You can get the three chest armor pieces and three closed helmets you need for the Cuman Disguise quest from that single chest, no need to raid camps or anything.
I’ve spent the last few hours in this game just going around knocking out minor side quests like the Charlatan, Hare Hunt, etc. and hunting down treasure maps and looting their locations. I am at the point in the main quest where once I take the next step, the endgame quest sequence starts, and I don’t really want to end the game yet. I know you can keep playing afterwards, but I just don’t want the story to end, so I’m delaying it by doing lots of other little stuff while rebuilding Pribyslavitz and beefing up skills.