I read the answer to this question
but somehow wasn't satisfied.
What's the difference between the 2 causative forms ～す and ～せる, e.g. 待たす and 待たせる?
Usually the せる form seems more common to me, but for some verbs, they both seem equally common, e.g. 聞かす and 聞かせる. Is that because the 聞かす I'm thinking of is not really a causative, but a transitive partner-word of 聞く?
The linked question lists a similar, but un-productive form, ～かす. Not the k from the stem, as in 聞かす, but a k in the suffix. 冷える→冷やかす, 散る→散らかす, やる→やらかす, おびえる→おびやかす. Are these fossilized instances of an archaic productive form, and does it have anything to do with causative?