Just going of how they chose to present the scene. He went back to a moment in time and effected it on a 1 to 1 basis, one pluck = one movement on the watch hand. Even in his tesseract of infinite time he would still need to perform them actions on a 1 to 1 basis and if the message was playing for decades he would be pressing on the ribbons millions of times. Would be tedious at the best.
I'm not sure where you get the idea that it's playing for decades and even if it is that means Cooper has to do it millions of times. He only has to do it once for it to happen simultaneously in all of Murphs timeframes. The tesseract is not subject to linear time.
You are imposing your linear thinking onto an expression that transcends linear considerations.
On another note if coop listened to tesseract coops message of "stay" where would that leave tesserect coop? Would he still exist outside that reality? Also would there be different realities, for example one where he chooses to stay and another where he doesn't listen and goes etc?
This one is simpler to understand.
Fifth Dimension Cooper occupies all time and space, every eventuality, every possible route and ending of mankind has already happened.
He only has to succeed once (think of it being one of an infinite number of possible outcomes, another Cooper stays, another leaves, another cooper dies on Millers world and so on) future humanity also exists outside of this fixed linear universe, so as soon as just one of the infinite Coopers succeeds in reaching the tesseract then that creates the pathway for Humanity to eventually transcend linear time and space and therefore exist outside of the constraint imposed by a paradox such as you suggest. It doesn't matter that in another timeline Cooper stays, because it only has to succeed in one for it to apply to them all as it is the transcended humanity that created the circumstances for Cooper to succeed in getting the singularity data to Murphy. Infinite failures are meaningless as long as there is a single success as future humanity exists outside of space time.
Imagine the universe (or multiverse if you prefer) is a glass globe, infinite timelines and infinite outcomes all expressed within that glass globe...humanity is subject to the rules and constraints when extant within that glass globe, but once they transcend and are no longer bound by the globe, they are able to look into that glass globe and they can see (and exist) in every moment of existence within it as they exist without.
The humanity that created the tesseract is ubiquitous.