Hello Beowulf.
It does sound like you're taking this good and steady, which is very sensible, given Willow's initial history.
There are a few threads on here which are recent, dealing with introductions, and on which you might find some useful hints and tips - the site swapping is something you're already trying, and plug ins such as pet remedy or feliway might also be a useful thing to try.
When we got Moray, we used Pet Remedy plug ins - one upstairs on the landing, and one downstairs in the living room, and whilst I dont know if the intros would've gone as well anyway, I dont think they did any harm, and I do believe they had a calming effect on the oldest of our two cats, who was also the flightiest.
You also have to try and relax when you do eventually give them some time in a room together, otherwise they'll pick up on your anxiety, and it will aggravate any nervousness. Make sure when you do try introductions, you have something handy to distract one/both of them in the event there's any perceived unpleasantness. It's often better to try and instigate play with a flying toy before it gets to a dodgy stage, as play will intrigue and engage both of them to a lesser or greater degree. Often if one cat is playing, the other will watch and see what interaction is going on. They may try and join in (always a good sign).
If you can, make sure Willow always has a safe place to retreat to, so her access routes are largely kept clear to begin with. If she likes to perch higher up, make sure she has available spaces to do so, or buy a bigger scratchingpost/play platform for her to explore.
Most cats will learn to tolerate each other eventually, even if its just to respect each other's space and keep out of the other one's way.
It's always a bit of a nerve wracking time, but in most cases, carefully managed introductions will pay dividends, and it sounds like you're handling this in a very intuitive manner already.
Keep us posted with progress reports.
Misty is beautiful, by the way.
