a) In the context of Tesla not using full hardware limit of batteries - this is standard on all modern low to mid personal vehicle diesels, they are very software limited on charge pressure/injected fuel from pretty early rpms. Otherwise, the high pressure turbo could charge the engine off the engine compartment. Apart from obvious legal (tax levels), or marketing-profit (price bins) reasons, it is done for the same reason Tesla does it. Running the engine in such "unlimited" mode, it would have about the same longevity as a laptop battery charging always to (chemical) 100.
b) I'm on mobile, cannot find exact models now, but yes, there were two engines having the same engine block&head, acessories, turbo, powertrain and were only tuned differently in ECU. Not sure now if it was for the same model of the car itself, though, maybe it was used across different model series. And a pretty common practice in European diesel cars with average power, loading ECU maps from higher versions - although often, the car would have smaller brakes or less Nm rated powertrain.
b) I'm on mobile, cannot find exact models now, but yes, there were two engines having the same engine block&head, acessories, turbo, powertrain and were only tuned differently in ECU. Not sure now if it was for the same model of the car itself, though, maybe it was used across different model series. And a pretty common practice in European diesel cars with average power, loading ECU maps from higher versions - although often, the car would have smaller brakes or less Nm rated powertrain.