pedropin.
Thank you very much for the feedback - I need feedback like this to make the plugin better... don't hold back

I can explain why M803 is not working.
The plugin only allows joysticks and button action, when the machine is idle and not in EStop.
This is done (by design) to prevent unintended interruption of a running program... and is the situation with M803 (Alt S).
This is a limitation on normal button usage and I intend to remove this restriction, but only for buttons.
DoButton 21 will not work because the machine is in EStop - It is possible to allow button actions when in EStop, but I'm not sure if this is a wise decision.
I plan to release an new version, when there has been more feedback.
Hope this helps.
- area51