In a campaign, if you have an ability that allows a friendly figure to recover strain, can you use it on an Ally? For example, MHD's bacta radiator, or some of the supply cards.

In the RRG it says that when a hero recovers strain, they recover 1 damage for each strain they can't recover. Does this apply to allies as well?

Also, can a hero use their special "Surge -> recover 1 strain" even if they don't currently have any strain?

Only heroes have Endurance. Other figures do not have Endurance.

Only heroes can recover strain. In addition, only heroes recover damage for each strain they could not recover. (Non-heroes cannot recover strain even if they tried. Only Recover damage applies to them.)

Yes, a hero can always recover strain even if the hero has not suffered any strain, either by the Rest action or through other Recover abilities, or their innate surge: recover 1 strain ability. (Note the lower-case recover. This strain is recovered during spend surges step. If the hero has not suffered strain, the hero recovers damage instead.)