That should be possible.
You'll need a check in each of your button commands to see whether one of these special modes has been requested (as there is no method for intercepting a command from another command).
"Let Go" could be as simple as using the "Make VoiceAttack Stop Processing All Commands" action, which will release any keyboard keys that are held down (regardless of whether they're being held down by VoiceAttack or not); It'll also stop any running commands, but your profile doesn't appear to have any where that would be an issue.
I will note the "Stop Sounds" action in your "STOP IT" command is both non-functional and redundant, as the "Make VoiceAttack Stop Processing All Commands" action will stop the command before that action is reached, as well as stop playing sounds and TTS (as mentioned in the documentation).
For the combo system, you could have the button commands append a text variable with the keyboard key they're intended to press, so the "Run Combo" command can process the contents of that variable.
If the intent is to press all keys in the combo simultaneously, hold them down for a set amount of time, then release them again, the "Run Combo" command could be as simple as using a single variable keypress action, which will do exactly that.
The "Build Combo" command would want to clear the text variable containing the combo.
E.G.
Rapid Fire
Set text [>function] to 'rapid fire'
Hold
Set text [>function] to 'hold'
STOP IT;Let Go
Stop VoiceAttack command processing
Build Combo
Set text [combo] to ''
Set text [>function] to 'combo'
Run Combo
Press variable key(s) [combo] and hold for 0,1 seconds and release
[Alpha ; Ay]
Begin Text Compare : [>function] Equals 'rapid fire'
Start Indefinite Loop
Press A key and hold for 0,1 seconds and release
Pause 0,03 seconds
End Loop
Else If Text Compare : [>function] Equals 'hold'
Press down A key
Else If Text Compare : [>function] Equals 'combo'
Set text [combo] to '{TXT:combo}a'
Else
Press A key and hold for 0,1 seconds and release
Stop all playing sounds
End Condition
If you don't want the "Let Go" command to release all keys (E.G. those for the joysticks), you can use individual release actions for the keys you do want to release (for the vast majority of applications, releasing a key while it's not pressed does not cause any issues), as well as stopping their associated commands, E.G.
Let Go
Kill command, '[Alpha ; Ay]'
Kill command, '[Bravo ; Bee]'
Kill command, '[Ex Ray ; Ex]'
Kill command, '[Young ; Yankee ; Why]'
Release A key
Release B key
Release X key
Release Y key