The current app I work on has a lot of warts, but only five `before_save` callbacks (there's maybe five times that many total callbacks, although there are lots of validations).

If you really want to standardize the interface, use the tools that already exist: ActiveModel and validations. Your “Interactors” are models; they just happen to be coordinating models that affect multiple ActiveRecord models simultaneously. (I did this for an account plan change system that affected billing and device assignment to a plan. It could easily affect a dozen or more records, all in a single transaction, but was itself easily testable.)