From what you've said re her weight loss and birth weight, I really would try not to stress about it too much right now. Bigger babies often (not always, but often) lose a higher amount initially, simply because being out of the womb uses more energy, and they have more to lose.
If giving the formula now is reassuring you, then by all means continue to do so - but it might be worth holding off until you've seen the lactation specialist (is it an independent lactation consultant, or a breastfeeding group?) before you make any long term decisions.
Either way, see if you can try and make some observations regarding the breastfeeds for now so you can discuss them - the things you've mentioned, plus roughly (no need to clock watch, just an idea) of how long she's feeding for each time, if she's taking both boobs, if there's any clicking, vomiting, nappies etc. They may not even want all that info, but if nothing else it might help you feel a bit better armed.
Tongue tie is very very common, but is often missed because it can come in so many degrees.
You sound like you're being really proactive, and you have nothing to feel guilty for. 