The fact your gearbox guy, as the expert, stated the problem noise was definitely a bearing and then charged you the lower cost of $1300 to do the minor bearing repair, as the ‘fix’, works in your favour according to Australian Consumer Law.
The fact you still have the noise post repair, with an added issue of the gearbox now popping out of gear, would indicate the gearbox guy isn’t skilful in his craft or simply didn’t take due care during reassembly. As such, the service he has provided didn’t achieve the results expected.
So, since he didn’t actually fix the gearbox fault, and in fact made it worse, the question you should answer is do you trust him to attempt any further fix? If the answer is yes, your gearbox guy should repair the gearbox correctly with all costs at his expense, including in my view the remove and reinstall of the gear box.
I’d be discussing the problem with the Office of Fair Trading, especially the issue of who pays for the gearbox removal and reinstall (since it’s no fun to have to do it again due to the gearbox guys incompetence in reassembling they gearbox).
If you don’t trust him to do any more repairs, I’d be discussing the problem with the Office of Fair Trading.
The OFT should be able to advise where you stand in relation to gearbox removal and reinstall and in making a small claim via QCAT with consequential damage such as gearbox removal and reinstall costs and possibly clutch and other costs...
As is, the gearbox guy, being the expert, advised you what the problem was and what’s the cost to repair would be, which you agreed to... he can’t, post his repair stuff up, say it’s now something else at fault and the repair will now set you back $5000 (because you need new gears, etc)....
Australian consumer law doesn’t work that way....
The normal ACL view is that he repairs the box at his cost or gives you your money back so you are no worse than before you did the shonky repairs and you are then be free to go to some other repairer. BUT in this case you are worse off post the shonky repairs as the gearbox now has a new issue of popping out of gear. That makes it a little more complex and QCAT should be able to advise.
As to the Oil, it’s not a solution to faulty bearings, worn gears or syncros... but if you thinks it is you try that before spending $1300 on bearings. Best to sort out the repair... talk to QCAT
Whatever you decide to do, don’t pay more for a fix that should been done and dusted for the $1300 the expert quoted and you paid...