I rarely use the tab and enter so I would agree that these could very easily have been moved to a shift function.
The only downside to this is the same potential problem with having
any shifted functions -- accidental activation.
IMHO it is better to have a keyboard (usually a bluetooth model) tucked behind the coin door or hidden nearby. YMMV.
It maps buttons to have dual functions, like holding pause button to exit, or holding player to drop coin.
Just as a curiosity check, some games like Omega Race have the option to start a multi-credit single-player game -- 7 ships instead of 3 in Omega Race.
Does your app wait until the button is released to output the keystroke or would a single-credit game start when you press and hold P1 Start to insert the second credit?
Scott