You mentioned that "If you pad your timers by 5 or more minutes, then you probably wouldn't miss the end of any shows... for all I know, that is how his cable DVR is set."
This is not true. If all three tuners are recording something, they will ignore priority and switch to the next program.
What are you talking about here?
IF all 3 tuners are recording something NO other timers can fire. IF you even try to set a 4th overlapping timer you get the "timer conflict" screen asking you how you want to resolve it.
Timers don't interrupt other timers.