Sorry to hear she was in the hospital... and you will probably hate my next comment as it may be considered distasteful, but the last time I used pay per view, I wasn't at my residence as I was away for a month and needed some fresh entertainment. Have you verified all the ways one can get a ppv movie charged to one's account with comcast such as watching it on a tablet at a hotel like I did (using one of my providers, not comcast). I'm not trying to say you/her did or did not watch or order the movies (I have no way to know for sure) just that the supervisor was more than likely right that just because you have a cablecard tuner doesn't mean you can't order content and that all their boxes do indeed have cablecards. The fact that this has been an ongoing problem states the reason why it is happening must be found out. Yes it could be a billing problem but usually that is a one time event (maybe two) after that it sounds more like you are just trying to get stuff taken off your bill that was more than likely ordered on your account (think Occam's razor from their point of view as they would never have a correct bill if it was that messed up, though I agree that the simplest explanation is billing error from your point of view). That is why you must collect evidence to how this can happen... what happens if a computer connects to your wifi and goes to xfinity.com or whatever it is. From what I could tell, pay options for watching on my computer are not available there but ordering options for watching on tv are however that may be that I am not on comcasts network, not logged in (as I currently only have cox, TWC and dish accounts) and not using an xbox or dedicated tablet app (I have read the xbox does have more options but am not certain) How do they verify all orders: login with username or email/password, use of a pin (while on your network... Since you don't have a box it may be difficult to change (unless its the pin for your bill like cox has rather than an order pin) , are cookies kept to keep you logged in (I support a number of public computers and though I don't remember ppv being a problem I know other login services can be when some don't log out)? I've even heard of extremes of "butt dialing" placing stock orders/sales so if your phone has an app that's another possibility. If you had an STB, I would even look at the possibility the cat or dog ordered it as I know some that love remotes. You already said no rugrats (crumb crunchers) in the house which is usually the easiest explanation (which is why I turned my suspicions, perhaps wrongfully, to your fiance and am sorry I can't totally dismiss the possibility). In looking at the xfinity site, I can see a create account link... the next page states "We were unable to verify your account based on your connection" which to me implies an account might be able to be created by logging on via your wifi... it also appears one can create an account using the last 4 digits of social, date of birth and phone number or even easier f they have your account number. Until you know all these possibilities and can dismiss all of them you may be on the hook.
My point is from their point of view you did order it and have a history of disputing orders so unfortunately its now up to you to get evidence you didn't. That is why I suggested posting in a comcast specific forum to find others that have had problems with billing and discuss how people can actually get these charges as it applies to comcast as I can only speculate as how their ordering and billing systems work... I've seen, attended demonstrations, sales pitches and used many over the past 20+ years though not all those were with cablecards and some weren't even for cable systems but satellite (which defines how most of my knowledge on phone/online purchases through third party resellers came to be).
Please keep us informed as I hope it is correctly resolved but to be honest, though it is probably what you prefer, a quick credit probably isn't going to solve your problem unless you also take measures to prevent any PPV orders (most systems have some measures to allow this but some have tried to charge a fee for it... I'm not sure if this is legal any/everywhere but I remember it being claimed that the reason for the fee was to keep the rates low by allowing the orders otherwise everyone would just block it and cut off the potential revenue stream). Believe it or not I'm hoping the best for you. I know it doesn't sound like it- sorry!