Also, the water doesn't need to be exposed to the Martian atmosphere - it can be stored in tanks until needed. Farms wouldn't be in open air anyway.

At a later point, it may prove feasible to reduce the incidence of solar winds using one or more deflectors at the Sun-Mars L1 point. A cool idea would be to slow down instead of deflecting so the particles would land on Mars and become part of the atmosphere instead of blowing it away.

But this is way past the science-fiction line for now.

A way to protect the atmosphere of Mars with a magnetic field has been suggested https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmo...

mars has an atmosphere even now. the processes stripping it off work on geological (or longer) timescales. if you can import (or free) enough water to make a difference in the first place, then keeping up with the losses should be pretty easy.

I'd assume we'd do it the same way we do it on the space station, except on a larger scale. We tightly control water consumption and what leaves the life support systems.