Not making any unusual noises?
tire not rubbing anywhere?
belt not overly tight?
if you can't twist it at all, I'd say it's way too tight.
your call.

That thing will adjust w/o a lift. As long as you can break the axle nut loose, yer in biz. I bet a dollar you can find someone in town who will bust the nut loose, let you adjust the belt tension & align the rear wheel to the frame, which requires no lift & then snug it back up for you. The axle doesnt Have to be incredibly tight. The adjuster is there & with a light load on it, the axle cant drift forward. The specs on torque are in the book & I cant remember it, but I really dont believe an impact is necessary. Take pics & E mail them to that shop. That was some crappy work right there I tell YEW....

If the brake plate is not in the slot, applying the rear brake can cause great damage. If it is just that adjuster that is wrong, the wheel may not be in alignment and could cause a handling problem.
At any rate the mechanic pretender who did the job needs to be called to account for a potentially hazardous screw up.
Come visit me for a while. I will give free assistance and education on the care and feeding of your machine.

Phelonius

If its not in, the you can either prove it by looking or rolling backwards & applying the brake, GENtleee... If it doesnt stop like it should, let off. Youve already proven that IF its not in the slot, its above it, so forward travel & using the brake should be okay. Youll have to get the axle loose to swing that adjuster back up.