What does "deviously" mean?

Most online dictionaries I've found define "deviously" as in a devious manner, but "devious" seems to have three meanings:

1. indirect
2. departing from the correct or appropriate way
3. deceitful, insincere

I find it difficult to apply any of the three meanings of "devious" when trying to understand "deviously" in the following quotes:

1. Flynn keeps the accelerator firmly to the floor, ratcheting up the tension with wildly unexpected plot twists, contradictory stories and the tantalising feeling that nothing is as it seems. Deviously good

2. The lawn still slopes deviously, and every day that I see it I am thankful for it.

3. Of course, it is summer, and the sun is shining, and it is deviously tempting to turn away from the void.

4. It's often funny, but at any given point, it's deviously unclear what we're laughing at.

How should I understand the meanings of "deviously" in the above quotes?

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.