Can "intentions" and "ethos" be used interchangebly when one is speaking about the author's literary work?
Your phrasing, "speaking about the author's literary work," is a little vague. Since characters are normally spoken of in terms of motivation and character development, I'm going to take it that you are speaking about applying ethos and intention to the author's presentation of a literary work. In other words and to paraphrase your question: When speaking about an author's ethos and intentions relating to a literary work, can "ethos" and "intentions" be used interchangeably? Let's start by definingg ethos and intentions to sort out your answer.
"Ethos," in literature, pertains to the moral element in a dramatic work that drives a character's actions, yet the author must imbue the work with ethos. (American Heritage Dictionary)
"Intention(s)," in literature, refers to the author's perceived end result or purpose in writing a work. (Collins Dictionary)
As examples of intention, you may say an author intends to expose a social problem, such as Dickens did, or intends to portray commonplace, everyday life in a city, as Balzac did, or intends to explore the 17th and 18th century significance of marriage in the upper classes, as Austen did.
As examples of ethos, you may say there is an ethos [moral element] of colonial criticism, as in Heart of Darkness, or an ethos of social justice, as in Jane Eyre, or an ethos of religious regeneration, as in Robinson Crusoe.
Now, to tie these two together, it is correct to say that an author imbues an ethos into a work that drives a protagonist's (or antagonist's) actions and that the author so imbued a specific ethos by deliberate intention. Thus these words are interconnected--the one is constructed by express design of the other [ethos is constructed by the express design of intention]--though they are not synonymous. In other words, you may say "ethos is imbued by intention" but you may not say "ethos is intention" nor "intention is ethos" nor "ethos and intention are the same" because they are not equal terms; they are not synonymous terms even though closely tied together by logic.
Ethos and intentions are quite different.
Ethos is the character of the speaker as portrayed in the work. It is divided into atechne (extrinsic) and entechne (intrinsic) ethos. Intrinsic ethos is developed through the attitudes and more values inherent in the speech, whereas extrinsic ethos consists of the outward fact about a person (family, wealth, past acts). Creation of intrinsic ethos is one of the three main forms of persuasion discussed in classical rhetorical theory.
Intentions are the internal reasons a character has for doing something. Sometimes they are made explicit when a character articulates his or her thoughts and at other times readers need to figure them out circumstantially. They are not related to ethos.