You are doing quite well. I'm impressed by your answers. I was expecting many of them to be more indicative of a less mature process.
Slurry: I do not recommend storing slurry for more than a week. You end up pitching too much dead yeast, and the yeast that you do pitch are in a very poor state. They may technically be "alive" but, they sure aren't healthy. For instance, look at the difference between how much slurry you have to pitch (based on Mr. Malty) when the yeast is freshly harvested vs. 4 weeks old. New = 87 ml, 4 weeks old = 187 ml. So, over half of the yeast material is now dead cells.
I make a lot of beer per year. I can ferment 4 beers at once in temperature controlled fermenters. I take very good care of my yeast (nutrients, O2, water chemistry, etc). I almost always use slurry the same day it is harvested. Otherwise, I start new. I have stored it for a week ONCE, because "issues" prevented me from brewing for a week. Even then, I took my measured slurry amount and pitched it back into starter wort the night before I brewed to ensure it was well fed, it got a new shot of nutrients, and was wide awake and ready to work.
oxygen (air): 10-15 minutes is good. But, I'd really recommend that you double that, to ensure that you are getting the full 8ppm that the yeast need. They really would like to have a little more than 8ppm, but that's the best you can do with air (not pure O2). So, go the full 30 minutes so that you KNOW the yeast have as much oxygen as you can. For big beers (1.070 and above) give it another dose about 8 hours later. This will help ensure that they have adequate O2 for the extra generation or so needed to finish those heavier fermentations.
Temperature: 66 is fine. But, you might think about warming it up after 2 days, by wrapping it in a blanket or sleeping bag. The yeast are producing heat as they work. So, wrapping the fermenter will help trap some of that heat. If you get it early enough (before it really starts to slow down, but after the growth phase) then this will retain some of that heat and help the fermentation reach desired FG.
When a fermentation slows down, it stops producing as much heat. Without temp control, it will cool off. This causes the yeast to further slow down, and if they weren't very healthy to begin with...they may just "give up".
FG: that's about what I kinda expected, a little higher than an All-Grain of similar OG. But, its been a long time since I made an extract kit. Fermentability of extract is highly variable from maltster to maltster. Some extracts can finish pretty sweet.
Airlock Activity: This is pretty normal after a transfer. It doesn't necessarily mean that the yeast have restarted. It CAN mean that, but often its just some outgassing of CO2 because you moved the beer.
So, what to do?
Your best bet is to warm the beer up by 5 degrees or so. There are a lot of ways you could do this, but the simplest is just to move it to a warmer section of the house. If you go more than 5 degrees that's okay, too. you are past the potential for flavor impact at this point. So, anywhere warmer is fine. If you can get it into the low 70s that would be perfect.
Then give it a week or so to do its thing, then see if the FG has dropped. You can go ahead and add your vanilla/jack concoction now if you like. It won't hurt anything.
To answer your original questions:
"what are the chances that things stalled? If it did, can I add more washed yeast into the secondary? "
Its definately behind the curve for 2 weeks. It should be at terminal gravity by now, even with the 66 degree fermentation temp.
Adding additional yeast IS an option. But, I wouldn't go that route first. I'd start with the temperature change I noted above. That has a very good chance of solving the issue, and it has zero risk. Using your 6 week old slurry has its own risks: dead yeast flavors (autolysis), and infection. If I went this route, I'd get a packet of US-05 and make a small (500ml) starter with it. Then I'd pitch that at the peak of its activity (12-18 hours after start, as soon as it builds a head of foam).
As far as adopting things "too early": That's fine. I always make the point because it can be confusing and frustrating in the early stages, and lots of people will give up if the beer doesn't turn out decent. But, as long as you understand that you are going to have "issues" that will result in learning "opportunities", you will do fine. Like this. Your result isn't exactly what you wanted...now you have a problem to solve, which results in learning a bunch of new stuff. Some of these "opportunities" may result in undrinkable beer. If you're okay with that going in...have at it. It is just a hobby, after all.
But, if you really want to learn more and make better beer sooner, then I'd recommend a slightly different course of advancement:
1. Full wort boil---no added water after the boil. Everything that goes into the fermenter was once boiling in the pot in one big pool.
2. Fermentation temperature control - you don't "hope" for a temperature you use a heater/cooler to force the temperature where you want it to be.
3. Yeast Starters - you grow exactly the right amount of optimally healthy yeast.
Oh, and SANITATION. I put this separate and last because it is not a "step", its a continual exercise forever. You never stop working to improve it. If you think your sanitation can't be improved, then you don't understand it. At every turn, every time you brew, look for ways that your beer could be infected and try and figure out ways to eliminate those risks.
Exposure of cold wort to any moving air, more contact with surfaces, transfers from vessel to vessel all expose your wort/beer to risks of infection. It doesn't matter how good you are, or how long you've been doing it. THERE IS ALWAYS A RISK when you move the beer. So, do you NEED to move the beer (why?)? Can you accomplish the same thing without moving it? Do you need to take the lid off or the stopper out? Is there some way that you can get a sample without sticking something into the whole batch?
Remember "sanitary" does not mean sterile. Every time your wort or beer touches a sanitary surface it could pick up a little bit of infection.
