With this script, Pause turns your hotkeys off and on. ScrollLock switches between the two key lists. (Your mode 3 is hotkeys off.)

I reversed the keys from what you described because I can do it faster and I have to go to bed. You can do it your way too but it's more work.

This is the whole script. As you can see it's quite short. If any of the keys control movement you can separate them out and use MovementHotkey for them, or you can just use MovementHotkey for everything.

Code:
<Hotkey Pause>
   <ToggleHotkeys>

<KeyList BigList AllMainKeys>
<KeyList LilList A, B, C, D>

<Label ....>
<Label ....>

<Hotkey ScrollLockOn BigList; ScrollLockOn Alt BigList; ScrollLockOn Ctr lBigList>
   <SendLabel ...>
      <Key %Trigger%>

<Hotkey ScrollLockOff LilList>
   <SendLabel ...>
      <Key %Trigger%>