If you cut the buds off a mophead hydrangea in the spring, you are removing your potential flowers for that season, hence the advice to be very minimal when pruning mopheads.

It’s doubtful that you killed the plants outright. It may just affect their blooming for next season. A better bet when you have diseased foliage is to rake up all of the fallen diseased leaves and throw them into the garbage so next season’s leaves are less likely to be exposed to the fungal spores. Typically hydrangeas survive fungal disease outbreaks without a problem. They are more of an aesthetic issue than a health issue.

You may not have an ideal climate or location for your plant. Depending on where you live, you may need to afford the plant some extra protection during the winter. Here’s an article that might help: https://farm-kits.com/how-to-protect-your-hydrangea-for-the-winter/

Whether or not to deadhead is really a decision for the gardener. It does not impact plant health or future flowering. If you like the looks of the dead flower heads, leave them in place. They’ll drop off naturally when they’re ready. If you don’t like the look of them, you can snip them off. Just be sure to not remove too much of the stem when removing the spent flowers.

Depending on your climate, you may need to afford it a bit of extra protection in the winter. Here’s another article that may help: https://farm-kits.com/how-to-protect-your-hydrangea-for-the-winter/