What does "deviously" mean?

Examples 1, 3, and 4 seem to best fit with definition 2: "Departing from the correct or appropriate way" and implies a moral judgment as well, i.e., we know a sensation should not feel good, and yet it does.

Example 2 (in the absence of context for the phrase "still slopes deviously") seems to best fit definition 1 "Indirect" which describes a physical deviation, rather than a moral deviation.

Your question is interesting. Dictionaries differ on their approach to adverbs. For example, the Merriam Webster online only defines deviously as the adverb derived from devious. The Cambridge Dictionary online, on the other hand, offers a separate definition for deviously which goes some way to justifying your doubts, which I share.

The Cambridge dictionary offers the following:-

in a way that is dishonest or tricks people, but is often also clever and successful:

This a much more subtle definition, of which every part has to be taken into account if it is to be used correctly. It illustrates an important feature of language: that The cognate words formed from a particular stem do not necessarily share its exact semantic extension. Any one of them can share some uses but not others.

The Latin origin of the verb deviate is the phrase de via meaning literally of the road or out of ones way. Its use as a verb is only attested in Lewis and Short's Latin Dictionary for post classical Christian use. The adjective devius is earlier and so used by Cicero in the late 1st century BCE, meaning off the high main road, and extended by poets like Horace from that to mean out of the way in the sense of remote or wandering about in unfrequented places. Cicero also uses it in the sense of personal the characteristic of being inconstant, erroneous, inconsistent. The adverbial form, say Mssrs Lewis and Short, "does not occur" (meaning, of course, that it has not been found among any of the writings that have survived and come to their attention).

The fact, however, that the Ancient and mediaeval writers did not use devious to mean dishonest or insincere does not prove that it cannot be used in that way now. Indeed, I do think the Cambridge dictionary is right to think that the adverb deviously is closely associated with this quality of deviousness. People who act deviously and so exhibit the personal quality of deviousness are definitely not to be trusted. It definition is very cunningly written to make it clear that deviousness is a quality cleverness mixed with a strong whiff of dishonesty.

the examples bear this out:

• He smiled deviously at Rob.
• The plot of the play is deviously clever.
• They argue that the operation is deviously designed to lower wages.
• She was behaving deviously towards him.
• The coaches would teach players to play deviously.

By contrast, the word deviant has a much tighter range of meaning, in the sense of having a characteristics or behaviours that deviate from social or psychological norms. Deviousness is a characteristic associated specifically with dishonesty, not to be trusted. Someone good at tricking opponents in chess or football, still remains within the rules of the game. A barrister who cleverly tricks the witness into admitting the truth is within the principle of proper law, and the strategy might be called "devious", except that it starts to make it sound as if there is something not quite right with what s/he has done.

We (I should say "I, I suppose) understand what the writer must mean, but something jars. This is how language works. It may be that this use will will spread enough for it to become established. Cambridge English dictionary will notice (at some stage) the change and revise its definition, so that my comments will be out-of-date. But for now, it is not quite right.

The SOED (Shorter Oxford English Dictionary) agrees with your three definitions. It puts the definitions as:

1. Pursuing an indirect or winding course; circuitous, rambling.
2. Deviating from the straight way, erring.
3. Subtly cunning, wily, unscrupulous, dishonest.

Examining the possible correlations of these definitions with your four sentences in question, I find these pairings:

• Sentence 1 with definition 3 (since the author may be wily or cunning in their crafting of plot)
• Sentence 2 with definition 1 (perhaps said by a character who prefers messy, complex images over neat and tidy ones)
• Sentence 3 with definition 3 (said by someone who, for whatever reason, prefers the void...morbid, pessimistic sorts of people)
• Sentence 4 with definitions 1 or 3 (perhaps you can see the connection here yourself?)