I’m a commercial photographer but have done a few weddings for colleagues. Its always seems to be the case that there is a nightmare lighting situation. This is not a good one!!
Forget HDR, to do it properly you have to take more than one exposure. Not something I would attempt with moving subjects.
Flash…possibly. But during a ceremony?! Its not always allowed. Depends on who is conducting it.
If you can get the couple to face the window that would be the best option. Use the available light. If that is not an option you will have to expose for the couple and have the windows blown out. This is going to suck as you could get a halo effect depending on how strong the daylight is.
Long cables are a pain. Get some radio triggers. Where are you based? I could lend you some stuff if you are anywhere nr leeds.
Have you done a recce? Looked at the venue etc? You could just tell the couple what will and will not work from a photographic perspective?