As an ex employer of at least 3 MOT testers in the past an MOT tester should not be stopping a test 15 minutes through to tell you it has failed.
He should be completing the test without stopping to comment on specific parts etc.
When a car is MOT'D it is logged onto the mot test centre and the tester then has a certain amount of time to complete the test and log the details, i think it is around 50 minutes, if he doesnt do this in time the test will be void, if he is going to stop and discuss every little thing he is unhappy about and get into a discussion then he could easily breach his time allowance.
I am assuming you failed in the end and need repairs ( in his opinion ) for it to pass ?
Obviouslly you will not be using that garage again, with regards to your seat belt on the child seat - just fyi you should not have ANY child seats in the car when presenting it for test as the mot testers are not allowed BY LAW to remove the seat belt in order to test its functionality incase it is put back onto the child seat incorrectly.
The car wont fail the test for this but they will usually put it down as an advisory.
As for his comment on the 18i wheels, what a twat,,, sounds like he was just jealous of your ride