I know that 415 limits don't apply to 457(b) plans (not that they would be relevant, since the 457 limits are lower), but for purposes of calculating the annual addition limit for the same employer's 401(a) plan, do you count contributions to the 457(b) plan? In other words, are these contributions "annual additions" for purposes of 415?

No, they are not considered as counting toward the 415 limit in the 401(a) plan.