it's a good question and in truth one that no one on here has answered with a factual answer, they are all answers from their own experiences.
From my own I have played around with the rest system all last season, putting players in rest mode is only beneficial when you don't want a certain number of players to perform one of the three training mode.
Putting them in rest for 3-4 hours in between or even overnight has no condition gain what so ever.
As you say Nordeus say you need not rest them as they get rested during maintenance, I'm at a loss as to what they mean, I selected 10 players at random over 7 nights duration noted all their condition and also the other 12, all had exactly the same gain the following morning.
Like most things on this game their explanations of rules and guidelines, are shall we say vague. Or just bollox, which ever you prefer.