IMO an oil change is a bit of a waste of money at this point, given that reverse works. The best it can do is give a vague indication of what's wrong by the condition of the old oil, but equally it may not tell you anything.
If you want, they could get an oil sample by pulling the small drain plug in the sump. You can remove that plug, get a sample of oil, and put the plug back easily without losing enough to make a difference. Dip some white paper in to the sample to get a good look at it. The oil should be a grey-yellow colour, should smell slightly fishy, and should be clear with no suspended debris.
Whatever it comes out like, there's no point changing it because its either perfectly ok, indicating a mechanical problem, or its not ok, indicating a different mechanical problem
If the oil is contaminated, the only way to flush it fully is with a specialist machine. Changing whats in the sump will only get about half of it so won't make much difference.
Tbh all the garage can do at this point is narrow down potential failures, which doesn't help you at all since it'll have to go to Mackies anyway.