Hugs love! There is NOTHING more frustrating than a kid playing games at bedtime AND getting up in the night. It's like you never catch a break!
At 20 months I'd almost be willing to bet money that canines are on their way in. Those are the most painful teeth, worse than molars. Kids start feeling them LONG before they appear. ibuprofin is a better medication choice than parcet. and also I'd probably alternate the 2 meds if you get more than waking (e.g. ibuprofin before bed and then first NW give parcet.)
I don't know if you've noticed this in the past, but when LOs are teething they usually start being more tired than normal. You have what sounds like a spirited LO who when exhausted just goes lights out at night (rather than having copious NWs). But just run-of-the-mill OT will cause her to have NWs. So, we need to kick that in the butt!
I realize she's had longer A times than most and has been on 1 nap since 10 months old, but 6 hours A time while also attending nursery is probably the problem -- at least when she's teething. I would try bedtime at 5-5.5 hours post nap and see if that helps. If 5.5 hours doesn't work after 3 days then go down to 5 hours and see what happens. I *know* this probably doesn't make sense given her personality but we see this behavior A LOT on the toddler boards and I'd say 90% of the time and earlier bedtime does the trick -- sometimes it has to be a whole hour earlier! Right now she's already OT which causes playing and then she's SUPER OT and just can't settle. Get her down early enough and that should help!
For NWs I would NOT do WI/WO right now. If she's teething she needs the cuddles and soothing you can provide. Also, it's just exacerbating her sleep problems. I'd also extend wind down -- add more stories, or other relaxing activities like quiet music or massage. That is another change we see at this age -- they often need a better division between activity and sleep and so lengthening wind down helps a lot! One thing I often recommend is reinforce the idea that EVERYONE is sleepy and going to sleepy -- even the outside world and inanimate objects. This is something that they're starting to sort of understand: that other people/things are like them! So, for instance, on your way to her room you might look out the window and say "Look! Mr. Tree is going to sleep now. Night-night Mr. Tree! There goes Mr. Bird on his way to bed. Night-night Mr. Bird!" Then when in her room, tell her stuffed animals night-night and give them hugs and kisses. You might even tuck them into bed. And then more cuddles with her before sleep. If you're used to just leaving the room after lights out, perhaps it's time to lay/sit next to her and just talk over your day a bit, maybe make up a story, or talk about what's going to happen tomorrow. Talk about how she needs her energy that she gets from sleep in order to play. One of my son's favorite things at sleepy times is for me to act out what will happen if he doesn't get enough sleep -- I sort of tip my head to the side, stick out my tongue, close my eyes, and got "aaaaaa" in sort of a dead tone. He thinks that's hilarious; then we talk about how sleep gives us energy to play. She won't totally get it, but it's good to help her understand what sleep is for.
She's a little young for reward charts; some kids can do it at this age, but many need to be closer to 2. But if you think she might understand a chart, make one up and put it in her room. Then you let her pick out a sticker that she gets to put on her chart in the AM if she doesn't get out of bed at bedtime and call for you. You can give her books to read in bed if you like -- let her feel like a big girl if you want -- but she is not to run out of bed. Another great reward is a hand stamp; toddlers love those!
Try the routine change and tell me what happens! Give it 3 days of course!