Word pairs like 嫉妒/忌妒 with similar pronunciations and meanings - examples and discussion please
First, as a native speaker I can tell you, the usage of these 异形词 is not dependent on regions or moods(at least to my observation), instead it is purely semantic or conventional.
Second, it is very unfortunate that, although some of the pairs are totally interchangeable, like the “嘟囔-嘟哝” pair you mentioned, but some are NOT. Some pairs of 异形词 DO have similar but different meanings, these differences usually lie in emotional tendencies, and some are used to describe different things, and in some pairs one word suggests different information than the other one does.
By the way, Chinese high school test paper makers like to use such pairs to baffle students, here’s a classic one:

煊赫(xuān hè) 烜赫(xuan3 hè) 显赫(xian3 hè)

They all say one is very famous, or one’s social status is very distinguished, the difference is the former two are somewhat disapproval, and the last one is neutral and most widely used.
You mentioned 直爽-直率,  they do have a similar meaning but they are used to describe different things. “直爽” is used to describe a kind of personality, “直率” is used to describe a style of speech (that is direct and frank). So you can say a “直率” way of speech is included in a “直爽的” personality. But yeah they can interchange.
Also, in some pairs, words carry different information. For example: 贫困-贫苦have a similar meaning, but the former one means only “poor”, the latter not only “poor” but also “live a bitter life”. 挣钱-赚钱 both mean “to earn money”, but the former one emphasizes “to earn money with labor, work and struggle”, the latter one means merely “to get money”. To this end, we say:

投资股票非常赚钱。Investing in stocks is very profitable.
我要努力挣钱。I must struggle to earn money.

In this kind of situation usually it’s OK to interchange but the result might look weird to a native speaker.
Some pairs mean very differently. In 不负(fù)众望-不孚(fú)众望, the former means “one did not fail people’s expectations”, but the latter means exactly the opposite: “one failed people’s expectations”.
Finally, there isn’t a set of rules to which one can decide whether a pair is interchangeable or not, but luckily in most cases people can understand you, and if you expose yourself enough to these ambiguities, slowly you can tell the differences.