I achieved my contest conditioning on diet and training alone... but I believe it comes down to the individual's metabolism, as mine is very fast...
I think the other two question come down to the individual as many variables come into play... Just a few could be daily activities, job, stress levels, metabolism, digestive health, immune system, sleep and sleep patterns, joint health, use of supplements and what one does during their training sessions...
I think the only way to tell is to monitor yourself very closely. Are you missing lifts, are you sleeping well, are you digesting food, are you feeling energized, are you experiencing serious fatigue... if you feel health start to decline and training performance suffers, I'd say it is time to back off, but as a temporary over-reaching strategy, not overtraining, I feel this would be fine... do doubles until you feel wiped, say 6 weeks, then decrease frequency for a few...
I recall doing doubles on big beyond belief and didn't take a break and just became so wiped out and fatigued I didn't even have the energy to get up and go to work... this was when I thought I was invincible and cruising wasn't needed... boy was I living in a fantasy lol