Doesn't sound professional at all. That is pretty scary practices for a so called wheel shop. Using the wrong seat on the bolts can cause loss of a wheel within a few miles- very dangerous not having them matched surface area from the bolts to the wheel sockets. They will not torque properly and IS in fact the reason the wheels drive with a wobble feel.

A plastic hub centric ring reducing the wheel center bore to match the hub is very common so you will be fine there. 5mm is still plenty adequate a hub lip for the ring to seat on and center the wheel properly in order to torque the lug bolts into place.

I actually replaced my lugbolts with aftermarket wheel studs and lug nuts so I did not have to worry about lug bolt lengths and them grounding out into the hub (thus potentially not torquing properly. Just food for thought(I have a thread on here when I replaced my wheel studs)