Hugs, I am sorry to hear that you are both going through such a rough time.
If you think that she can handle the bigger teat - it might be worth persevering with it - it can take a few days for them to get used to it. Try feeding her a little more upright when in your arms to help her with the quicker flow. At least this way she is getting a bigger feed.
With regards to the teething. It can take absolutely ages. My DS started teething at around the same age as your lo, and his first tooth only cut through when he was six months old. The teeth start moving up into the gums and this can be very uncomfortable for them and this is why they refuse their feeds. It will definitely be worth trying some teething gel to see if that helps soothe her gums.
With regards to the nap problems, it sounds like your lo is ready to start increasing her A time. Are you following the EASY routine? If you are, putting her on the 3.5 hour EASY will help her transition to a longer A time as well as longer time between her feeds. With her staying up a bit longer, she will be that little bit more tired, and should help her sleep better.
Have you had a look at this link with regards to transitioning to a longer EASY?
https://babywhispererforums.com/index.php?topic=63161.0Hope this helps.
Paula