Hi there!
Just wanted to offer my support as I've been where you are now just a couple of months ago. I know it's not nice, so lots of hugs to you!
Accumulated OT builds up over time and can take a while to get out of too. My DD is now 19.5 months and she also got OT during the 2-1 switch, in the midst of teething her molars, then getting an ear infection then a throat infection
Illness obviously doesn't help things.
What are you doing when she cries when you leave the room? Do you stay til she sleeps or do you do WI/WO? I would suggest WI/WO as she'll then not get used to needing you there to settle but you're still showing her you're right there, just not staying in her room when she falls asleep. The other option would be gradual withdrawal where you gradually move further away, over a few nights, til you're out of the room. SA can get worse with OT and especially at this age I've read. It's then important to show her you're there but without creating to many bad habits.
The way we finally got out of the OT was by managing to push the nap later and getting a longer, better nap in and then did some earlier bedtimes for a while (appr. 4.5-5hrs A max) but basically stayed consistent for a while and she then adapted and caught up.
Also if she is teething a lot, try giving Ibuprofen at night (lasts up to 8 hrs) if you haven't already tried it. It would at least help her sleep longer if she's waking due to pain, but EW are sort of part of the parcel it seems
It IS a very messy time during the switch (at least for some bubs) but you will get through it. It can just take some time, especially after a cold too. Maybe try earlier bedtime on the days she's done 1 nap and a normal one on a 2 nap day?
Just want you to know I felt just like you 2 months ago and now things are much better, so remember it won't last forever. You're not doing anything wong, this is just a phase she's going through. Stay strong and make sure you get your own sleep as much as you can to keep up your own energy levels. It will pass!