I'm not a dad but my husband is a stay-at-home dad. I hope you don't mind me responding - I wanted to because your situation sounds familiar in some ways. I'll try to go issue by issue.
First, from what you wrote I feel very strongly that your marriage is worth working on and keeping. You've been together for many years and the kind of distance you feel now is very common once you have kids. Marriage can get a lot more difficult once there is a child, and requires a lot more conscious effort on the part of both partners. I think you can be happy and fulfilled and stay in your marriage, with some effort and love. One of the best gifts you can give your daughter is a happy marriage - so even if at times having your happy marriage means your daughter has a babysitter or something, or you otherwise feel she's "missing out" or might be missing her daddy, remember that.
You talked a lot about sex. It's normal for sex drives to not really line up anymore after you have a child, for a lot of reasons. First, if your wife is breastfeeding, that can severly impair (to the point of totally killing!) her sex drive. She may feel insecure about how her body looks and feels after having had a baby. Also, your schedules are more "off" - your wife wants to go to bed earlier because she's tired and has to work, but your down time is after your daughter goes to bed, so you're up and she's sleeping. The practicalities of your life are making it harder to be excited about going to bed together. And I don't know if this is part of the equation or not, but I think that your living arrangement would make me feel somewhat awkward. I know there's a floor/ceiling between you, but I would be more anxious all the time about someone hearing something and that would make it harder for me to get in the mood. Not sure if that's part of the problem, but it occurred to me.
I think that there are definitely ways to deal with the sex problem. Since both of you agree that there isn't enough of that going on, the question is why and how you can both work harder to make it a reality. It may be that what your wife really wants is for there to be attention paid to her, and for her to be focused on, in order for her to feel like sex. I think you should have a very frank and open, non-blaming
discussion about this particular situation and try to figure out what would help her feel more interested. It may also partly be the disconnect that you feel between your lives. Working on reconnecting in other ways may really bring your sex life back to life also.
While I think it's admirable and wonderful that you want to devote yourself to the SAHD job 110%, don't forget that you're a husband, also. I suggest a regular date night (once a week would be good). You have convenient babysitting right downstairs (right?) or you can hire a babysitter. You can go out for dates or stay home and just have time for the two of you to talk, reconnect, and pay attention to one another. While your wife may have to work consciously at it to some degree, I think that sex often naturally flows from a situation where one feels appreciated, noticed and valued by your partner.
If your wife isn't breastfeeding and is getting enough sleep and still has no sex drive, you may want to look into possible medical causes, such as depression. I think also that maybe a thyroid problem can cause a lack of sex drive. Obviously I'm no expert - I haven't had to go down this particular road before. It doesn't sound to me like she doesn't love you or doesn't want to have sex - just that she doesn't have any desire. This is pretty normal after a woman has a child, particularly if she's breastfeeding and doesn't have the return of fertility yet. 19 mos is on the older side of this, and I personally feel that while there is often naturally some break in sexual activity after a baby is born, as long as everyone is healthy it's incumbent on both partners to work on making the sexual relationship healthy also (rather than just ignoring it) if there's a really extended dry spell like the one you're describing.
With regards to your daughter, your wife just feels insecure. I can relate to this - I've been known to say stupid things like "Does she even know who I am? I'm worried she won't even know I'm her mama." As a working mother, I can tell you it's very, very difficult to leave your baby at home (even with her daddy, who you know is the best possible person to care for her), and that is made even harder if your baby then shows a preference for someone other than yourself. It can be absolutely heartbreaking. I think it would help to talk to her about how she feels about your daughter's preferences and remind her that it is not indicative of how much she loves her mother. Does your daughter ever ask for her mother during the day? If so, I would call your wife at work if you can and put your daughter on the phone, saying that she asked for mama and you wanted her to hear mama's voice. Help your wife bridge this pain, insecurity and separation anxiety she's having. Encourage her to take your daughter on solo outings on the weekends so that they can forge a stronger bond and develop their relationship, and use that time yourself to refresh yourself and do something like a hobby, visit a friend, or work part-time - just to get yourself out or doing something meaningful to you.
Being a SAHP can be isolating and can lead to depression, even if the circumstances are good and there's no "reason" to be unhappy. Sometimes just being cooped up and feeling socially isolated can do it. So find ways to get yourself out and keep some interest alive, even if on a smaller scale than before, that you had before your daughter was born.
Sorry that was so long. Obviously, we've faced many of the same challenges, but with a lot of good communication and some work on both our parts our marriage is happy and strong.