No offence taken, I'm just getting the impression you don't want to do anything unless it's 100% certain it needs it. None of us here (that I know of) will say you need to do this or that. A boat is just like anything else, it needs maintenance to keep it worry free. We don't have to change the oil but it will last longer if it's done.
There are two grease fittings on the engine coupler, and if you get your gun on those which are behind the motor and in front of the stern (tight spot). Greasing this area may or may not stop the squeak.
The motor may not need the impeller changed or the alignment adjusted. If the impeller fails later and the motor is shut down before it overheats, it can be replaced then. If it gets to hot, then your probably pulling the motor or heads and fixing the issues. If the engine alignment is correct and has not moved all is good, if it isn't later, you will be pulling the motor to fix.
Without pulling the drive you don't know if the gimbal bearing is bad. If it is, it can be fixed later when it comes apart or causes more problems.
In your previous threads you have trouble with the steering, so this could be in part to what your hearing, maybe not.
The stuff mentioned is not that hard, it just takes some time and one special tool.
Everything is a choice, were here to help if you need it