Hi.
I'm not an expert but if she pulls off after let-down, you may have a strong let down reflex, which can upset some babies. I would recommend a call to your local Association of Breastfeeding Mothers bf counsellor, or the NCT helpline (both have websites).
Certainly, your milk supply will adjust to accomodate the formula top-up, but if you are not feeding less, then it shouldn't dry up. If you want to kick the bottle altogether, you need to do it soon as yopu you are ready to, and drop one yop up feed at a time. Expect just a few fussy days as she re-stimulates your milk supply.
Are you on a slippery slope? Only if you choose to think that way. You might consider only bf for most feeds and adding one bottle in the evening, that works for quite a few people. Many babies fuss in the evening, as oxytocin levels and therefore milk production is at its lowest then. It's perfectly normal, and a shame more mothers aren't warned early on.
Please remember that formula feeding isn't a BAD thing- it may not be many peoples ideal option, but once you're past the colustrum stage, it should always be remembered that there are others ways to 'compensate'- I had to give up feeding my first son after his weight plummeted below 5lbs, so I made sure his early diet was full of oily fish and veg, and all feeds involved cuddles, regardless of how delivered.
And yes, I did breast feed the next child.
It sounds like you are doing a wonderful job of trying to care for your little one in the best way you can. Be guided by her weight, and what works for YOU not other people; take skilled advice if necessary and I am sure you will continue to be such a caring parent. You have a very luvky baby, that you are so concerned.
ps sorry about spelling- bf my ds as well as typing! good luck