Do you think I should present the user with a warning in the GUI if I suspect they're doing something odd - like assigning a long press to a direction?
Sounds like extra work for you on something that won't give much, if any, payoff.

As long as there is a way to reset back to default and/or clear the input you should be good IMHO. Picture MAME's "Input (this Game)" menu -- enter/escape clears the highlighted button, enter/escape again resets it back to default.
Regarding the switches. The dedicated switch is allowing for people to attach on/off switches rather than momentary push buttons. I should refer to this as on/off switch.
An on/off toggle would be possible by using a single press, a double click or long press of a momentary switch.
Regarding auto-fire. When auto-fire is set, the device will send pulses to simulate rapid button presses.
I currently have a delay setting of 25ms between pulses.
I like your suggestion to adjust the speed:
It would be possible to add configuration for the auto-fire speed (delay between pulses).
This auto-fire config could be set in the GUI and assigned as a function to buttons. So you could select a default delay of 50ms and then assign button inputs to speed up or slow down the auto-fire rate.
OK, this makes more sense.
I think the word you're looking for instead of "dedicated" or "on/off" is "continuous", since that is the industry standard term.
Sounds like your Customizing GUI will have 8 columns and 3 radio buttons/checkboxes.
-- Based on your earlier screencap since you haven't sent a link to the beta version yet.
Nudge. Nudge. Wink. Wink. Know-what-I-mean?

This would give you the following list of columns, functions and associated switch types:
1 - Pin (A1-A10, B1-B10)
2 - Momentary - Momentary pushbutton (Default function)
3 - Long Press (and maybe very long press) - Momentary pushbutton
4 - Double-Click - Momentary pushbutton
5 -
**Toggle - Momentary pushbutton
6 - Autofire - Momentary pushbutton sends pulse train.
7 - Autofire Speed - Pull-down box with the numbers 1-9 (9 is fastest/default)
8 -
***Continuous - Light switch or latching pushbutton
**=Should this cell be greyed out until you select a radio button/checkbox after Momentary, Long Press, or Double-Click cells? May need two columns to define toggle "on" and toggle"off" states.
***=Will this be a continuous "press" which might interfere with bootup , a momentary pulse on state change, or a recurring pulse at regular intervals?
I'll play around with that online metronome later to find a decent range for autofire speeds.
Scott