Sigh... I wish I had looked properly at the written tutorial you have before posting this!
Firstly I had read it that you needed an eCart store to apply this to, and second I missed the fact you already have a video explaining most of what I have just asked for... so thanks for the video, very enlightening.
I think my problems with validation stem from trying to use DW's built in validation behaviours as I didn't realise you have a tool of your own. So many little gems lay hidden under that '+' button!
I do have a question to ask, so I don't know if this thread should be moved from the wish list to Data Assist, but here goes.
You are putting your validation on two different pages, one is a confirm page and the other is a checkout page. So, before I go through the rigmarole of duplicating pages and multiple undos, do I need two different pages to implement this sort of validation, or can I apply it to the same page the form is on? The data assist or form builder pages seem to have it all on the same page as the form, but I might be wrong.