In December, I saw a couple of MG HS being repaired (bodywork, denting, painting) at Toyota Ravi motors, Lahore. I asked their service manager about it later, he said they were being serviced there earlier, now they have their own workshop and get their work from there. It might be the same for twin cities for now I guess.
As for your other points, I have been to service bays of Toyota and Suzuki several times, without any issues or anyone asking me. In fact at most dealerships, you take your car directly to the service area yourself and tell them what work you want done. They do have it written that customers are not allowed sometimes, but it is generally not followed, and customers are not stopped from visiting the service bays.
One thing I have noticed is that if you have been a customer of the big three and have visited their workshops, you will not like the service bays of new entrants. The scale of the big three service areas is too big to be achieved by new entrants easily.