I think the mistrust that has been there clealy still affects things. And of course this text from that woman at work says something to; I must say that telling someone you love them is quite a big thing and I'm not sure that a completely one sided crush could have got to that point without there being some encouragement from him.
I think you need to sit down with him and tell him that his behaviour looks rather suspicous (I take it he didn't come home with the video in question?) and that, after the affair and the rather dodgy text, and the funny story (with no video) you've found it quiet difficult to trust him and so on and so forth and see where you get to.
Although, with all that said, if he has a mobile then why use a payphone. That seems like a bit too much of a faff if you ask me.
Anyway, I think that even if he is telling the complete truth, the fact that you don;t trust him means there are some underlying things you need to try and work on with him. It's all well and good "forgiving" him for having an affair but if you can't move on whole heartedly, it kinda seems pointless.
I like Pink's idea of trying to get in touch with the woman who texted him though. See what she has to say.
Assuming he's told you he couldn't hire the video, have you try ringing the video shop to see if they have it in stock, btw? If they do, you know he's talking b*ll*cks.