Does the chengyu 话不投机 (huà bù tóu jī) mean "uncongenial speech"?
話不投機 means you don't share similar interesting or thoughts to chat with people or people don't like what you talk about, think it is not interesting.

Regardless the sentences of example you give is true or not, I just consider the semantic since I am not psychological professional.

发现话不投机，好好聊天才是捷径 show that people have the property of 话不投机 so on the latter part of sentence it encourage people to 好好聊天.

话不投机比不相爱更可怕 shows that 话不投机 is more horror then 不相爱.

许多话不投机 shows that the couple have a lot of different belief.

话不投机半句多 shows that you feel that 半句 is enough if the chat is 话不投机(you feel 半句 is a large amount since it is undesirable).

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