It can be very difficult to do the right thing when you have a beautiful, but invasive, plant to deal with! Here's a few thoughts:
1. your father is right that a clematis montana, which is what this is, can and will do damage to a roof, lifting and displacing tiles. The stems get thicker every year, and a stem under a tile will thicken and push it out of place. Having spent a large sum last winter removing clematis from the front of our tile-clad house, replacing damaged battens and rehanging tiles, I've got rid of that clematis even though it looked glorious when in flower.
2. You can cut the clematis back quite hard, removing everything on the house, and keep the part that is on the pergola. In future keep it confined to the pergola or perhaps let it scramble through trees. Cut it back as soon as possible after flowering, because it will flower next year on growth made this year. You will inevitably have fewer flowers next year.
3. If you just try to fold back such a huge mass of clematis I think you'd end up with a great ugly mess. You could try it, but you'll probably end up having to cut large lumps off anyway, and once you've started trying to rearrange it you'll find it much harder to cut only what you want to cut.
4. I don't know how you could stop the tendrils of the clematis getting into the attic again - they find the tiniest crevice and exploit any gap. In my view you'd be better keeping it off the roof in future.
Sorry if this isn't what you want to hear! However, once the job is done and the clematis has settled down, it won't be the disaster you fear. Good luck with it.