Mhmm, the reading is a different story. This is most likely a 漢文 taken from Chinese literature (
http://zhidao.baidu.com/question/15580584.html, but it is a guess only) and I reckon it will have a more poetic (traditional) reading, like やまかいのあき. Although the reading you are giving is also correct.
山峡之秋 is unlikely a name of a person, and it probably describes the painting itself, or explains its ambience, but again, only a guess on my side as I have not seen the painting.