The use of in-laws with the word 'law' pluralized is incorrect.
On the contrary, it's correct in the sense that in-laws originates from an abbreviation of parents-in-law (you may substitute any appropriate relation for parents here). You're right that we pluralize the noun (parent, sister, etc.) rather than the adjective. But when abbreviated, the abbreviation itself (in-laws) becomes a phrasal noun functioning grammatically as a compound noun, and we pluralize the ends of those. This usage is so common that in-law isn't even an abbreviation any more; MW, Macmillan and others list it as a noun in its own right, following the standard pluralization rules.
Your examples aren't quite analogous, because they all end with (or simply are) nouns. Instead, I'll turn to one of the prototypical American English pluralization gotchas: attorney general. This follows the same rules of plurality as parent-in-law because general is an adjective, rather than a noun, as with in-law. That means it becomes attorneys general in the plural. A common abbreviation for attorney general is AG. And if I'm speaking of more than one AG, I'm talking about multiple AGs.
ONE PAIR OF PARENTS-IN-LAW + ONE PAIR OF PARENTS-IN-LAW = PARENTS-IN-LAWS?
No, they would still be parents-in-law (or in-laws if you omit parents). We would understand that there are multiple married couples of in-laws from the context. If you want to be explicit about it, you'll need to add a few more words; as I've done in the previous sentence, for example.
As an aside, in your game show example, we'd be likely to just say parents, because they're only in-laws to half of their respective married couple. But of course that has nothing to do with your actual question and I know it's not what you meant.
To my (American) ear, "What would most people know about insulin?" is correct, and "What would most people knows about insulin?" is incorrect.
"What would the most people know about insulin?" sounds unnatural. "The most people" is usually used as a superlative, not as a way of referring to a specific group of "most people". "Most people" is deliberately vague as to which people are in the group, so a non-superlative use of "the most people" is usually self-contradictory.
"What is the most people you have ever talked to at the same time?" does sound natural. Notice that this question asks for a superlative -- the result of comparing the sizes of all of the groups of people "you have ever talked to" -- so this use of "the most people" refers to a single group of people. "What is the largest audience you have ever spoken to?" and "What is the largest group of people you have ever talked to?" sound even more natural.
"The most people I have ever talked to was 2,500 people, during a meeting at Town Hall." In this usage, "the most people" is singular.
Best Answer
People is a collective noun. When we talk about a specific group of people, we consider it as singular and therefore, no need to add s.
Peoples is used when we talk about two or more different ethnic groups. For example, "All the 14 distinct peoples (native groups) of the continent were part of the survey".
Whereas People's is not the plural form. The apostrophe ('s) is used to indicate possession.
Likewise, the People's President means the President of the People.