You still have the same problem as option (a): you're asking the PC clock to be as accurate as the timer. And of course for short periods, it will be, but we know there will be drift between the timer and PC clock. You could re-sync the two periodically, but doing so within the event would mean varying degrees of dissimilarity between timer and PC clock across the various results.
I'm not sure why you'd want to trigger an event manually when you just wait for an athlete to create an impulse by starting or finishing, but there must be a reason or you wouldn't be asking here. In cycling, time trial start lists post a start time in hh:mm:ss for each athlete, but we don't assume they started on time and just record TOD for their finish and calculate the difference. No, we take exact TOD of both start and finish. Group start events like road race or XC, we still take an exact TOD of start rather than banging in the scheduled one.