The low levels of gravity (microgravity) in space cause significant changes in astronauts' eyes and vision after six to 12 months aboard the International Space Station (ISS), according to a study published in the IEEE Open Journal of Engineering in Medicine and Biology.
If the time frame you will be required to be out in space exceeds the time it will take for your eyesight to degrade to a point where it hinders you physically being able to do your work, then that is a problem you can't just soldier through. There will have to be some sort of solution to this.
It's actually a good idea, provided the monkeys can survive the zero g environment and feed themselves and clean up after themselves and whatever else. Free return is something like 16 months?
Hahah, just create an entirely new system architecture compared to what is currently being built. Easy! Set back human exploration timelines by decades...
What is currently being built can't sustain a monkey population to mars anyway, so why not? If the monkey trials are a priority, we need to give them suitable living conditions