The buds of my drift rose are covered in white powdery substance and not opening. Occasionally, part of the bud is eaten away

I can answer part of your question (I hope). The white coating looks like Powdery Mildew. It's relatively easy to combat. You can simply spray it off with a hose or a spray bottle or you could use something stronger. If you search this forum for Powdery Mildew (PM) you should find many options and opinions. Some of my roses get it annually - it's ugly but it doesn't kill them.
Part 2 : what's eating the buds - I don't know but there are many here, including some in your area of the country that will probably be able to give you ideas! Best of luck to you

Powdery mildew. Mine got it for the first time this year after having lots of drifts for 6 years. Neem oil knocks it back. & the powdery mildew Doesn't seem to bother their blooms. Nothing seems to bother drifts, really. I'm gonna let them finish blooming their little heads off then I'll cut them 1/3 back so the mildew is gone and they'll carry on just fine i think.

Thanks! Naturally, I already cut most of the blooms bc I didn't think it was powdery mildew bc the leaves weren't affected. Next time I'll know! Thank you!