Oh OP, you can't only look at the things she does that support what you want to hear, and fob off all the things she's doing that challenge it.
Unfortunately yes she can absolutely be enjoying your company, your commitment and the fact you actually want to be with her, whilst still being very attached to this man she clearly does not want to give up. As others have pointed out, you are filling the gaps in her emotional needs, obviously she wants you to keep doing that, so she will reciprocate the sentiments to sustain it.
You need to focus more on the other facts here which are that she does not want to give him up, or she already would have, even though she knows it's not conducive to a good relationship with you ( or anyone, its just common sense).
That she is someone who thinks its ok to have an affair, and doesn't care about the impact of that, on his wife or their family, or on you.
This isn't about what a prize she is in your eyes, that he must have felt like he'd won the lottery etc is all really just rose-tinted glasses. If he had wanted her that much he'd have left his wife wouldn't he, not spent a decade struggling to work out whether she was worth doing that for before deciding she definitely was not.
It also isn't about who on paper is better, you or he- She has you on a plate and is still thinking about, and engaging with him, when she possibly can, and sharing personal emotional information (probably about you some of the time) in efforts to sustain her bond with him. She might 'choose' you in your hypothetical scenario but she doesn't actually have much choice really, since she can't 'choose' him as he isn't actually on (full) offer. If you leave, she has nothing, hence why she doesn't want you to, the heart still will want what it wants.
That doesn't mean she doesn't like you or like things about you, of course, but it certainly means she doesn't like you enough.
10 years is a long time. Do not be fooled into thinking that her saying she will immediately drop it, means it will be the end of it. Look at the lengths she has already gone to.
You should not be this full of doubt and jealousy and comparing yourself to another person in a 4 month relationship. For your own sake don't take her agreement to take the photos down or 'stop talking to him' ( aka, stop for a while, or just hide it from you, she has form for lying remember, a 10 year affair) as proof of her love for you. Being in love can do funny things to us, but listen to the sensible people here.