Sounds to me like you don’t need pheromones for this as she already is showing interest, so why use pheromones to see if they work on her if she’s already showing interest? What good is that going to do but have her continue acting the same way she’s acting now, or if you’re lucky maybe open up a bit more. However, as mentioned why use pheromones when you can have her open up even more just by you being you as she’s already showing interest?
Also I know every guy on here will think otherwise depending on their own values, but if you were to ask me why even bother? What are you going to get out of this but a possible lay from someone who’s already taken, isn’t of class as she’s obviously engaged but showing interest in another guy which is indicating bad juju, and a possible confrontation with a pissed off fiancée. Really, not even worth it. I know that when a guy is horny they don’t care, but you obviously stated that you’re a good guy so if I were you I’d just rather show her that I’m better than that and move on or just keep it friendly.