I'm sure Traya could utilize her assassins in other ways. Sneaking them aboard Republic shuttles bound for capital ships. The utility these assassins have is great. And know that while the element of surprise would be lost after one or two instances of assassin encounters, that is an advantage in itself. Knowing that there are assassins lurking does more damage than the assassins themselves would do. Paranoia, sleep depravation, and other things would occur if they know Traya has assassins to use against them. This causes a whole host of problems for the Republic. Suddenly, you don't know what to do, you can almost feel them standing behind (even though they aren't really there). That alone can lead to Revan losing.
Traya, wise as she is, will no doubt make use of Malachor V to prevent Revan from fully defeating her. If Revan attempted to engage her at the Trayus Core, he would fall to the Dark Side. The Dark energies of Malachor, combined with Traya manipulating him, would cause him to join her, or even take control of Traya's forces. This gives Traya the victory.
I would agree with you that Revan COULD NOT beat Traya one-on-one at Malachor V
. But he wouldn't have to do that.
Let's review the points we've made:
Revan's army is likely larger then Kreia's
Revan is a better leader WHEN it comes down to legit battles
Traya will likely use her assassins rather then get into a legit battle
Traya will likely stay at Malachor V - where Revan couldn't beat Traya in a duel do to the whole darkside nexus thing
Revan has a significant logistical advantage. Remember: he's commanding the Republic army - which (obviously) is owned by the Republic. The Republic is FAR more capable then Traya's empire when it comes to trade/resources and stuff. The Republic has been around for thousands of years, while Traya's army has only been around a few years. Traya purely commands an army, not (many) citizens. Citizens = money. Money = good.
So in a prolonged war, Revan's army will be much better off then Traya's. In fact, instead of engaging Traya at her base (Malachor V), Revan could simply wait/starve her out. So in a prolonged war, I believe Revan wins it due to logistics.