The MitoQ supplement is promoted as containing a particular form of Co-enzyme Q10 which is well-absorbed by mitochondria, unlike ordinary Co-Q10. Is it critical that people take this special form, or is that more a handy marketing tactic?

This is an effective form of Co-Q10 but it is not the only way to benefit. As you reasoned, Co-Q10 does indeed work, so it must be able to access the tissues and cellular compartments to exert its biochemical properties. This is simply a function of dose level.