re 2) my impression so far is "guard" behaves similarly to "defensive", but could not notice the difference. Looking through aiprops, sections related to behavior, probably "guard" might act similar to "aggressive" but stay inside UFO/near the entrance. But this is a very rough guess
maybe you can shed some light on one more thing: in ufocontents, a particular alien person is coded like "2-2 Sebillian_Guard_Aggressive". I'm still guessing what's the purpose of the 2nd number? I haven't seen it differ from the 1st one, which has to be the spawn quantity...