Being a super fan of the series, I'll answer the best that I can.
I don't think Neo's(Reeves) version of the Matrix was the only one where peace could be established with the humans and machines. (In this context, peace is only referring to the characters and world in the movies, it's a stretch to even say there is peace if you include the video games).
I do think the machines are prepared of a number of situations all of which will benefit them or in which they have control. Remember when Neo goes to the machine city and meets the giant machine, the machine loudly says
We don't need you! We need nothing!
The whole purpose of Neo was to give humans false hope that someone with extraordinary powers will come and save them, and to use his code to reset the matrix. For the machines, this is the ideal situation.
The Architect states
They're levels of survival the machines we are willing to accept.
In this situation it's a world without humans.
Now, other situations can have consequences the machines are prepared for but probably want to avoid, because it could severally damage the foundation of the matrix world or take up a large amount of resources that they simply don't have or can't reuse right away.
why would the previous versions of Neo (who is known were not in love)
ever chose to go back in the matrix?
As pointed out by the by the Architect, Neo's love is far more specific(Trinity) while the other Ones's wanted to save everyone. The other One's had no real reason to reject what the machines wanted them to do. Up until Neo, the other Ones's proceeded accordingly, because the Matrix is still there, and people are still searching for the One.
what would have happened if Neo would have chosen to go to the source?
How would have the machines handled Smith in that variation?
Same as always, the Matrix gets reset and admin control or total control goes back to the machines, and then they can eradicate him(much as they did at the end of Revolutions). Remember, Smith had a little more control than previous versions after he attempted to assimilate Neo, as he pointed out, a piece of code could have been imprinted, over written or copied. Given his new found powers, he probably found a way to prevent the machines from deleting him.
Because Neo didn't go to the source, the matrix couldn't be reset and Smith is now a virus.
Or, the other way around, what would have happened if Smith was really
defeated and Neo went back into the Matrix to save Trinity? How else
could he have saved Zion?
If Smith is really defeated and Neo instead went to save Trinity, I doubt the machine couldn't proceed, I believe machines had a backup plan (probably insert the code themselves or create an emergency program to do it for them), but the ideal situation is for the One to choose to save Zion and the cycle is repeated over again.