It sounds like you are in the middle, and you are right to be worried about what your place is as a friend.
Aeron and his fiancee need to have a lot of direct communication. It sounds like they are not talking to each other, and that each of them is talking to you instead. If they are not on the same page as each other, this relationship will not work out. You say they have lots of love for each other, but it doesn't sound like they have a lot of trust or honest communication. And you are right to worry, because it is common for people to blame the friend that they put in the middle!
Also, you say you are getting stressed out. While it is very cool for you to care about your friends so much, I think you need to take a step back and see what your role is as a friend. It sounds like you are supporting Aeron in his transition, and it is nice for you to give him a safe space to present as male. I don't know what you mean by motivation, though. I don't like how it sounds like Aeron is saying you are his motivation to keep transitioning, because it puts a major responsibility on you that doesn't belong on a friend. You can be encouraging and supportive and a good listener and provide a safe place for presentation and use the right names/pronouns that Aeron wishes. But at a certain point, his journey must be his own, and he must be self-motivated to do it. Furthermore, I think it's important that you tell the fiancee that you recognize Aeron as the gender he asks to be identified as, and refer to him as such - without sugarcoating it.
In a nutshell, it's not your job to keep Aeron on the path to transitioning and it's not your job to block him from it either, and it's not your job to figure out what Aeron's gender is and it's not your job to tell his fiancee how to act either. So please do your best to be supportive, to encourage the couple to work things out together directly, and don't feel stressed or guilty about not fixing everyone else's problems. You sound like a very good friend, and I think it is cool that you wanted to come here to get Aeron help. At a certain point, though, you will be a better friend by having Aeron deal with his own stuff by himself, rather than through you.