So, forgive my ignorance of how your particular civilian trade worked, but how could you, as an unlicensed mechanic, train jouneymen and apprentices? Would they not need to prove they were working under a licensed supervisor?
That aside, despite the list of training accomplishments and experience you provided below, in order for you to get a favourable result from the PLAR, you will have to demonstrate a direct link between your training and experience with the job requirements and skills taught on the veh tech courses. For instance being the service manager and having the certifications to use lots of gucci kit is hard to equate how you will be able to work at the Pte/Cpl rank with our limited equipment, on SMP vehicles.
If the meat is there though in your file, and the direct links are made, you should stand a good chance of getting some advanced standing/quals granted.
Best of luck.