Does a unit with negative Might contribute negative damage to my Might total in combat?

No. Negative Might is treated as 0 Might for the purposes of dealing damage, but as its true value for raising it back above 0. So a unit with -3 Might would contribute 0 damage in combat, would die if it was dealt 1 or more damage, and would require +4 Might to get it back up to 1 Might.