Check the following:
1. front-wheel bearings, some lateral-play between brake-pads when QR is barely on. No play when QR fully tightened.
2. head-set bearings. While standing on side of bike, squeeze front-brake and push handlebar back and forth. Movement here can also show front-wheel bearings, so do #1 first
3. rim clean? Use some acetone to wipe down both sides. Oil and grease really isn't visible on the braking-surface, but can lead to slippery spots that cause shuddering
4. brake pads clean? Wipe those down as well, and pick out any metal. Maybe even lightly sand them with 200-grit sandpaper to remove any smoothness
5. RIM-WIDTH, use calipers to measure the outside width of the rim. Slide the caliper along the rim to see if it catches anywhere along the way. Impacts into potholes, speed-bumps, etc. can cause pinched-flats, and also spread out the rim sidewalls a little, leading to grabbing and shuddering brakes.
And yes, find another shop. Their responses are very shady, leading me to think that they know about a pre-existing condition on that bike.