One common misunderstanding is the difference between U-boats and US fleet subs. The U-boats all had direct drive - a clutch on the back of each diesel connected directly to the propeller shaft. The rotor for the motor/generator was wrapped around the propeller shaft, and was used as either a motor powered by batteries or a generator to charge the batteries.
A US fleet boat had no connection to the propellers at all - each engine was connected to a big generator, and nothing else. The traction motors propelled the sub both on the surface and underwater, there was no other means for turning the shafts.
If you think about the difference between reciprocating engines (especially diesels) and electric motors, the reciprocating engine has a lot of back and forth up and down movement of pistons and valves - over rev a piston engine enough and it will shake itself apart and/or explode before it overheats.
An electric motor is very simple in comparison and has only one moving part, the rotor. Traction motors back then normally used between 500 and 750 volts volts DC, but if you had a way to feed 3000 volts into one you could wind it up like a berserk alarm clock for quite a long time before the insulation would start to melt. Obviously you would need to short around the fuses and clamp down the contactors, but a traction motor would spin happily at 10,000 rpm for an hour or two if there was a way to feed enough power into it. You'd need to overhaul it after that type of abuse, but it would be unlikely to fail.
The real limitation would have been how much power the diesel generators could pump out, and of course the increased drag of the hull moving through the water. The trick with using the low pressure blower and leaving the Kingston valves open to make a stream of bubbles under the hull sounds viable, dunno how much it would decrease drag but it would decrease it.
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