I'm sorry that you're dealing with a difficult situation. I don't have any experience with asexual marriages but from your post you do sound conflicted. I think it is possible to do what you're suggesting but I also don't think it's the only path. It might end up being that you and your wife will transition into being friends or something else entirely. In your post, you go back and forth a few times being having a poly relationship and stepping out of it entirely. The one thing that sounds clear is that you are interested in poly regardless of what else happens. There is no right answer for what to do with your wife, only the one that works best for you. I would imagine that it would be very difficult to live with some one that you very much desire to be sexual with them but to have it always be off the table. That imbalance would concern me. Even if you get to have girlfriend who is sexual it still won't mend that imbalance on it's own unless you really are at peace with not being sexual with your wife.
I think that any one involved in any kind of alternative relationship has to prepare for the possibility that people can find out and probably will eventually. And sneaking around does suck. I was in that situation previously and it really wore on me. If you like to live in the open, that is something you should consider. Some poly relationships are very much on the down low and some are really open. There's a whole spectrum. You have to examine the price of admission to be with your wife and decide if that's what you want. Don't stay in it for the sake of your families. But if it's where you really want to be, then go for it.