Just to make sure, you DO have this same exact noise whether in Park or Neutral OR sitting still with foot on brake pedal and in gear correct? If the same noise and volume of noise is present in any gear position and sitting still with your foot on the brake then it is likely servicable without remove the transmission completely which means removing the side cover should point to a problem. Once you pull the lever into reverse or drive with your foot on the brake pedal and not moving then nothing inside the trans spins as far as geartrain goes, so all that leaves is the pump shaft and pump.... you mentioned it didnt seem to get louder with load so we will not worry about the converter at this point. Now that "when" has been established and our possibilities I answer you questions in order

1. With your mileage there is likely a bit of wear on everything, but as mentioned above only your pump and pump shaft is spinning and working in gear and not moving, which you mentioned still exhibits the same noise. I very rarely see pumps fail in these transmissions unless a lot of metal is in the systme from internal damage. One thing I do see from time to time is a bearing inside the pump rotor in poor condition or falling apart. This will certainly cause noise and also tear into a sleeve/support that is pressed into the valve body and used for a guide for the rotor to sit on. When this gets pitted it will always make noise. The pump shaft itself more-less just floats and there isnt much that supports it or could cause noise, but he splines on the converter side will wear over time and should be replaced if worn. If the above is OK after apart and inspected you are left with the pressure system. The pcm controls the pressure control solenoid inside the trans and if it is bad or failing it can cause high line pressure which will also make the pump whine. Internal wear and overall low line pressure will cause the pcm to raise line pressure to overcome slippage from wear over time and if it is out of its acceptable limits then it will go into limp mode with max line pressure.

2. As mentioned above due to when the noise is present this problem should be able to be fixed just removing the side cover

3. The only obstacle is you will need some type of engine support to do this as you will have to hold the engine and trans up while lowering the support craddle from under the front of the car. Sometimes you can get away with just loosening the passanger side cradle bolts and completely removing the drivers side bolts and allow everything to relax at an angle to get the side cover off. Other than an engine support bar the transmission itself isnt going to require any special tools for what you can get into through the side.