TrisG
An operator can change the major modules of the gearbox on the "line" (i.e in a normal level maintenance facility as at Aberdeen), but you cannot disassemble the gearbox and inspect every component as you describe, for good reason however.
If you were to visit a gearbox overhaul facility you would be amazed at how sterile the environment is – it’s a temperature / humidity controlled "white coat and gloves" zone which is forensically clean. You would not wish to take a gearbox apart in anything other than that environment because in so doing, you introduce way more risk than the benefit from completing an inspection. All of the Aberdeen operators use specialist facilities to complete this work, two of them use Eurocopter.
The gearboxes and their components are managed closely, and the component “lives” (how many hours the components can operate for before inspection or retirement) are set at very conservative levels. The TBO (time between overhaul) for the EC225 main transmission is 2000 flight hours and given the average aircraft utilisation in Aberdeen , that’s an overhaul, and inspections just like the ones you describe, just about every year.
Hope that gives you some comfort.
Scots.