Learn English – What’s the accepted way to use “criterion”, “criteria”, “criterions”

As pointed out in your question software is a mass noun also known as uncountable. Some uncountable nouns can be used in plural, and Google n-grams reports a number of instances for softwares:

"These softwares permit simple mapping of the existing data and offer good compatibility with other softwares." (Don De Savigny, ‎Pandu Wijeyaratne - 1995)

"In addition, these softwares, although said to be "symbolic" fail to provide explicit motion equations of the considered system." (Thomas Lee - 1993)

"Many microcomputers are equipped with softwares built on ROM." (The Malaysian Agricultural Digest - Volumes 2-3 - Page 48)

"All ten softwares have been written in Turbo Pascal for the IBM and IBM compatible computers." (Mark Keegan - 1995)

"the communication between the two softwares uses a common memory zone." (Philippe Geril - 1986)

"Dozens of well-developed softwares for computer-assisted molecular design based on 3D—CG are commercially available and are now widely used (8)." (H. Timmerman, ‎T. Fujita - 1995)

"The Decision Making Software is a set of softwares to make control decisions of the agent." (M Ahmad, ‎William G. Sullivan - 1993)

"There are many commercial data mining softwares available in the market that can be applied to financial applications." (George T. Albanis - 2003)

The question arises whether this use of softwares is the result of the influence from non-native speakers of English or simply the natural evolution of its use. This is a question especially difficult to answer for technical terms such as software.

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.