After several weeks now I've gotten to the point where I can stand outside the doorway with the door open until she's asleep
I know it doesn't feel like much, but you've made progress so that is great
It still is taking a long time of wind down to get to this point and this is the same whether she naps or not.
what kind of wind down do you do and for how long are you outside her door before she nods off? do you have the impression that the winddown helps and is needed? if it doesn't seem to be making much difference, perhaps try shortening it or taking some bits out, to get her into bed sooner?
Now you're outside her door, could you try waiting a little bit further away each night? I think it really is a big step that she now nods off with you outside of her room and out of sight (is that right?).
Regarding the NWs, since you've made such good progress at BT I'd try the same with NWs, e.g. do WIWO when she's upset and tell her you'll wait outside her door like you do at BT. Like you described with dropping the co-sleeping, it can take a rough night (or maybe a few) at first whilst you establish the new norm, but if you're consistent she should start to accept it. And with WIWO she will know that you do come and comfort her when she is upset and needs you.
What is her mood like during the day? Does she show signs on SA ATM? It can be a common time for it, with both or either of developmental changes and teething being the culprit. Could there be any teething (canines?) troubling her which could cause the NWs?