You are definitely not a terrible mother and you have not ruined everything. Yes, it's best if you can as consistent as possible in avoiding APOP during a period of sleep training, but it's always most important to keep you and your baby safe and sane.
Looking at your routine and the scenario that you describe, what stands out to me is that in your ideal routine, your LO would be getting 15 hours of sleep in a 24 hour period. My kids (especially my first) tended to be on the lighter side for sleep needs, but by 8-9 months, they were only needing 13.5ish hours of sleep--my first more like 13 hours of sleep.
We went through a very frustrating period of sleep training when DS was around 8 months. He'd been sleeping terribly--no routine, hit and miss naps, 3-6 times a night nursing back to sleep. We found BW and started pu/pd. Night sleep improved immediately--one wake up the first three nights, then STTN. We had a few days of good naps and then naps went down the tube. No matter what A time I tried or what trick I tried, he was not napping more than 45 minutes to an hour once a day. After days, I decided to take a break from day time sleep training and APOP by taking him for car rids for his naps. I'd had in my head that he needed 3-3.5 hours of day sleep and in the car, I could get him to do that. Well, two or three days into our APOP for naps, he suddenly quit STTN and we started to have long (1-3 hours) night wakings during which he seemed like he would have been quite happy to get up and play, but was going to scream if we insisted he stay in his crib.
This went on for days and finally I sat down with my sleep logs and did some math. I realized that no matter what, he was only getting about 13.25 hours of sleep in a 24 hour period. If I forced the long naps with APOP, then he was creating A time for himself with long night wakings. If he slept a solid 12 hour night, then it was very difficult to get him to take more than one nap (which he needed for his mood).
Once I realized that he only needed about 13.25 hours of sleep, I was able to work out a routine that reflected his actual sleep needs. It meant dropping to a 10.5 hour night and having much longer A time than I thought he could really handle, but we shifted to it gradually it ultimately it was successful.
Later in his life, when he was down to just one nap, we went through a phase where he'd shift back and forth between having 13 hours of sleep and having 11 hours of sleep in a 24 hour period. It took me a while to realize that he really needed about 12 hours and he was getting a bit too much one day, so then doing a bit too little the next day to balance things out.
In the first few days of our sleep training there was 2-3 days where he slept well at night AND took decent naps. I realized in retrospect that he was so OT when we started that he needed a little extra sleep to catch up. Once he was caught up, his actual sleep needs emerged and they were simply less than I had imagined.
All this to say, without knowing all of your history, if you've been sleep training and had 5 nights of significantly increased sleep, it's possible your DD was catching up from being very OT. Now that she's caught up, her actual sleep needs may be emerging and they may be a bit different than what you imagined.
If I were in your shoes, I'd personally look at gently stretching A time and reducing day time sleep. I'd probably start waking her at 1 hour from her first nap, and plan on less than a 12 hour night. This is just me.
When you get the long night wakings, what happens if you leave her in the crib and let her play and just lie on the floor beside the crib? While you're sorting out whatever the cause of the NWings is, it's important you handle them in a way that won't perpetuate them. Light plays a critical role in establishing wake/sleep patterns. So, if your LO wakes in the night and you get her up and expose her to light or Television, you are signaling to her brain that this is a good time to be awake and will make it more likely that she wake up again at the time the next day. If possible, when you have night wakings, I'd try not to focus so much on getting her back to sleep (I know easier said than done), but rather just keep her company in the dark in the least stimulating way possible. So, I'd make a bed on the floor, lay down, and wait. If DD is distressed, I'd verbally reassure her, maybe sing or hum softly, perhaps stick my hand in the crib (although this gets uncomfy fast) and just wait. If you can, fall asleep yourself.
Do what you can to preserve your sanity. Knowing your nights may be crazy, see if you can get DH on board with some sanity time for you during the evening or on weekends--long bubble baths, an hour to slip out by yourself and read at a coffee shop, etc. I found for myself that it didn't have to be big or long events (although those are nice too), but an hour here and there throughout the week could really help me keep perspective.
It's possible that there's not a good explanation for why your LO is doing what she's doing. Sometimes there just isn't. When that stuff happens, all you can do is handle it in the lest destructive way possible and hope the phase passes quickly. {{{Hugs}}} I hope something her helps.
