Good surmise, but only in the case of !important. Otherwise let the CSS cascade handle the rest.
Yes, but sometimes not that easy to find depending on how they embedded the CSS. In the programming world we try to use "best-practices", which if used correctly would make it easier for another programmer to come along after the fact. However, in the world of modifications they are seldom used. Many modifications like here on vbulletin.org the programmer has offered his talent for free, so the extra compatibility effort is hardly worth it.
Every developer/member that have offered free modifications here, are golden, and have my highest level of respect for their time and effort.