I have experimented and found the most straightforward way to marry Arwa is to accumulate right to rule before starting the rebellion, by sending out companions or being a mercenary/vassal of a faction that gains a truce while winning. I found 21 (but not 18) was enough R2R.
I cheated in this rebellion to win all the battles, and when I took fiefs I allocated roughly half to Arwa and half to me (Arwa always took my advice about how to allocate, even when I experimented with taking all for myself). I didn't recruit any lords at all, just wiped them out so I don't know whether you would have similar results if you were allocating most to lords but half of the remainder to Arwa and of the remainder to yourself, but I suspect it would still work. When the rebellion was won, all the lords defaulted back to the faction and I was Marshall. Then, before I had a chance to accumulate any controversy, I proposed and she accepted. As co-ruler I handed out most of my feifs. Notable that Arwa still had too many, but you could a) take them all for yourself originally then hand them all out as co-ruler, hoping you don't run out of or b) allocate them rationally to defected lords as you go, but make sure Arwa and you are roughly equal.