Thema: In a relationship with a person who has alexithymia

Hmm, asking a group of people who are unable to process emotions to interpret the emotions of someone they've never met in something as complex as relationships / cheating may not get you very far.
But here's some food for thought if it assists you -
- did you ascertain why he cheated on you? If he has no emotional response to the cheating, was this what he was trying to provoke in the first place? Psychopaths have been known to behave in extreme ways in order to trigger their emotions, as normal stimulus won't achieve that. An alexithymic may have behaved in a similar way. If he still has no emotional response to the act though, it seems his test failed.
- you seem to be asking yourself if he loves you or not, basing your enquiry on his demonstration of love. I know many alexithymics can't love, so it's possible he does not and is just paying lip service to the word so as to perpetuate the relationship for other reasons (habit, utility, ethics, etc) and knows it is what you require. Some alexithymic people however can love, but may just be very confused about what is happening, unable to understand, and in many cases unable to communicate that through traditionally acceptable ways that would be perceived by you as a demonstration of love. His definition of "love" and yours may differ. He may indeed feel for you what he believes is love, but it may differ from your understanding of the emotion.
- it seems contradictory to tell you he misses this woman, but also does not think of her unless you mention her. Which would mean he only misses her when she is mentioned. I know many alexithymic people who are also autistic don't "miss" others. They're either there, or they're not. The social emotional attachments required for pining are sometimes not present. As to why he told you he does miss her however, seems unclear.
- whether you continue with your relationship or not is entirely your own decision. However your post reads as if some more clarification and understanding is required between you to really understand where he is coming from, what his emotional inabilities actually are, and whether or not you decide you can work with them or not. If the therapist knows enough of alexithymia to suggest it to him, perhaps they could meditate between you, to help you better understand his emotional differences and whether or not he is acceptable to you as a partner with you aware of those differences.

The reason it got worst after is not because it got worst inside him, it's because he doesn't have to maintain the veil of illusion he was putting forward as the gig is up.

Emotional attachment is kinda hard for us, we might love someone or not... or sometimes it is a confused mess intermittent and all... doesn't mean we don't feel that the person doesn't have an importance. If he didn't had anything for you, he would probably just leave. And yeah as the previous guy said, people are there or they are not... there is no longing pain in between, or if there is, we won't be able to identify the source unless we have learned to deal with part of it (after some therapy mostly)

If you want an allegory about how emotions are for us, well it's a spectrum, from no emotions, to: I'm recently blind and I try to feel the world through touch only and I'm not sure what I'm touching. I feel bumps and cracks, fluffy and gooey... but I'm not sure what it is.

Truth is, your needs, you'll have to spell them out for him if you want to stay together and be patient with it. Put clear rules with a rational backing them will make it easier for him to follow them. At least imo. And don't wait to ask directly, without holding a grudge about his inability to anticipate, when you need something. If you can't do that it will never work.