Yeah I went and tried the highwaymen once in my previous game and with that stealth mechanic I was like, nope that ain't happening. Maybe if you can eventually gain enough stealth to reliably go first, but other than some very rare encounters where I randomly get lucky, like picking the lock on the door in that first town hall quest, I have never seen a stealth increase.I've not seen the Highwaymen of Darkmere in a long time. Once they started firing first I became a little more wary about going up there, but eventually they disappeared entirely. I don't know if it's related to the bandit count or what, but eventually I stopped even looking for them.
Regarding power katanas, I forgot to add something about them and the arena upthread when you asked. You do need to equip your girl with one when you send her to the Arena. Then she will have one available. Most of my girls fight with fists outside the beginning matches. I usually start with one that yields 400-600 and see how close the score is, then play around with the weapons and a higher-tiered opponent to get an autowin with about a 10 point cushion. I've found you can still lose even if the numbers add up to 100 so you need a little extra over that, about 10 points is enough in my experience.
I played around quite a bit with different options in the arena and yeah I did eventually figure out the power katana secret. I think you can also have your slave use one in the arena even if you don't give her one as long as you have one in your inventory that isn't equipped but I could be mistaken on that. I'm not willing to take stat losses in the arena so if she has less than a 100% win rate I pick a different opponent.
I find that the valkrie can consistently always beat all of the heroic opponents with a weapon right from the start with no training. She seems to always have at least 2 combat relevant perks in my game. Unfortunately Rebecca can't consisistently beat anyone including the $100 ordinary opponents with fists in my game. So if I put her in there all she does is beat up the ordinary opponents with a weapon for $50 which is actually a loss when you count the entry fee, although you do still get influence.
So what I do now is grab the Valkrie as soon as I unlock Crystal Hills and put her in there doing the heroic opponents until I unlock Aria. Aria is almost always weaker than the valkrie in my games due to bad luck on perks but she can consistently beat the veterans with a weapon in all of the games I've tried so far. So once I get Aria I alternate between Aria and the Valkrie in the arena and whichever one of them is not in there gets daily training in strength/dex/melee. The goal being to eventually get the valkrie trained up to where she can always beat raven with a weapon. Not sure if it's possible to get her to where she can always do it with fists (she can some of the time without any training but not consistently).