I spent another hour separating out more of the code, so that the Timebomber is now almost completely independent from the Bomber. This fixed all bugs except one:

When the Timebomber countdown reaches 0, "BOMBER" is still displayed during the ohno/explosion phase, the Bomber explosion graphic is used, as is the Bomber destruction mask.
My current codework and gfx content
wants it to read "TIMEBOMBER", display a different explosion graphic (made purely for testing purposes), and use a much bigger destruction mask (again, made for testing), but something is clearly still amiss.
To further clarify what's currently happening:As far as I can see, the code is saying: Assign Timebomber > Do Countdown (during which any skill except Timerbomber, Bomber and Freezer can be assigned, and during which the lemming will continue to do any activities they could previously do such as Climb, Swim, Float, Bash, etc) > Enter OhNo! phase (during which no skills can be assigned) and display "TIMEBOMBER" on the skill panel when moused-over > Blow up the lemming, display the "Timebomber" sprite graphic for 1 frame, and leave behind a destruction hole using the Timebomber mask
The actual output is: Assign Timebomber > Do Countdown (during which any skill except Timerbomber, Bomber and Freezer can be assigned, and during which the lemming will continue to do any activities they could previously do such as Climb, Swim, Float, Bash, etc) > Enter OhNo! phase (during which no skills can be assigned) and display "BOMBER" on the skill panel when moused-over >
Blow up the lemming,
display the "Bomber" sprite graphic for 1 frame,
and leave behind a destruction hole using the Bomber maskSo, for all I know the entire Timebomber process is actually just the Bomber but with a timer added, which is
not what I want; I need it to be a separate skill so that I can debug it as such. What I don't understand is why the program is seemingly ignoring the Timebomber code, or at what exact point it's no longer reading Timebomber code and is instead reading Bomber code. Need rest and reflection, I'll come back to this another day.