Resists on the ground function the same as in space. They'll help prevent loss of hp, not shields. To protect shields, you need shield damage reduction.

They stack the same as in space as well. There are no diminishing returns based on number of sources. 10% + 10% resists is the same as 20% resists.

That said, the actual damage you take is dmg * (1/(1 + raw_resists)). This means that, as far as damage reduction goes, the first 50% will prevent more damage (1/3rd of raw damage) than an additional 50% (it'll prevent an additional 1/6th of the original damage, for a total of 50% reduction). This is what people typically refer to as diminishing returns.

Worth noting though, that a certain percentage of resists will perfectly counter act the same percentage of bonus damage. 50% resists means it takes 50% more damage to kill you. 50% damage bonus means you kill in 2/3rds of the time. And 1.5 x 2/3rds puts you right back at 1. So, basically, if resists have DR, so does damage.