About the text and being ignored. Would you want to have him answer a text message from his other partner if he was spending time with you?
It's just the basic things of thinking it from their shoes. Would you be ok if they did this or that. That can also kind of help to see where they are coming from also.
It might not be the best idea that you are getting such a slice of time specifically. I do think it is a good idea that you and he have personal time to each other where you can be together and really connect again. It might not be easy to have just that one day and as for the shared day
If you can make it work, it's not so bad but since you are in a V and not a triad, it might be a little odd at first to have a day with his other lover and expect any real kind of intimacy.
These are all just my two cents and I am probably not the most experienced person to ask about this but hope it works since it is pretty apparent that you do care for him.
And a little bit on what you said about the fighting it to the bitter end, I would personally find that a miserable way to live my life. I personally would equate it to being gay and yet doing the straight thing of getting married having kids, etc. fighting against that side of me. Again, this is just my opinion and I'm not saying you should do this or that, but good luck.