Will "chante to..." skills benefit from synergies?

[adeyke]

Regardless of how a skill is "activated," if you have base points in the synergizing skills, the skill will be synergized.

So yes, the Bonehew on a necro would benefit from base points in bone spear synergies.

Your merc doesn't have any points in bone spear synergies, so he won't get that benefit.