Figs defend at 1:1 in space.
on a citadel, if they are offensive, they attack at 2:1
on a citadel, if they are defensive, they attack at 3:1
Unmanned ships defend at 1/2 their defensive value. Tholians (or any ship with the defensive bonus) only get the bonus when they are manned. There is nothing I can think of that will defend an unmanned ship other than q-cannon and sector figs. In a nutshell, if you leave an unmanned ship over your planet, it's meat.
See
http://www.tw-cabal.com/formulas.html for the answers to your questions. See entering a hostile sector and landing on a hostile planet.