"Forget about frame by frame. Do we measure the speed of a racing car at Salt Lakes frame by frame. Of course we don't"
Yes, actually you do - if you are for example trying to see who won.
You want to ignore the frame by frame analysis because it PROVES YOU WRONG.
"If at 30 bars a minute we have two seconds for each bar, four beats. Then if I dance a Slow as two beats. There is something wrong if I can't get the next two Quicks on the remaining two beats."
Anybody can do that walking or dancing. Can't they. But if I am a competitor my musical expression takes over.They do play around with the music to suit themselves. Don't they."
Which means that in fact, no skilled dancer does dance the slow as two beats. But they do dance the SQQ as precisely four beats.
And there is your mistake. You see Sinkinson or Gozzoli drawing out the prep step into the first measure. If you were to do that, you would still try to end your second quick at the end of the first measure - you would shortchange it and have danced the prep step plus the feather as QQQQ.
But that's not what the video proves them to be doing - instead, if you count the frames, you will find that they draw out the third step of the feather exactly as far into the second measure as the prep step carried over into the first measure.
They give the three steps of the feather step four whole beats, just not aligned with the barlines. But if you dance the same prep step they did, you would cut the feather's three steps short so as to still end on the barline - a very beginner mistake.
"The original thread simply quoted from a lecture given by Irvine in which he taught to accentuate the first quick. Then suddenly there you are frame by frame. Later it was added that if we get the two quicks on the beat with the music the slows will look after themselves."
Actually you are putting things out of order. You quote Irvine's comment, but you also added a wildly inaccurate guess about what it meant. You were correctd about that immediately

You then brought up the beginner idea of putting both quicks on the beat, and were corrected about that.
Only then did we get into the frame by frame analysis measurements - to PROVER WHY YOU ARE WRONG - they show that the time between the quicks is at least a beat and a half, and in some cases almost two beats.
"I think I might have difficulty not stepping on the beat."
I think so to - and it's clear this is why you still can't understand foxtrot. Foxtrot is not about stepping on the beat, it is not about stepping at all - it is about moving the BODY in time with the music. You have to stop thinking about the moving leg and start thinking about the BODY, or you will never get beyond the level at which you have so long been stuck.
"Incidently Billy Irvines first quick and Len Scrivener's thoughts seem to compliment each other in the Foxtrot. Don't they."
To a degree, yes.
If you'd give up on your mistaken desire to put the second quick on the beat, you might eventually learn this dance.