After attempting any possible method of inputting the correct name(as filename, configured name in ruleset file, and .json added) of the ruleset which i would want to set as default, it won’t change. Even if the name contains no spaces or non characters. After briefly reviewing the code i cannot figure out where the cooresponding method to set this value is even used, it seems to be implemented but not called.

Further, the documentation for this file is vague and weirdly formatted

It's been working for me. As an example if your json starts with:

{
  "Name": "My Mod v2",

In this case you should just need to modify the MelonPreferences.cfg file in your (possibly) C:\Program Files (x86)\Steam\steamapps\common\Demeo\UserData directory to appear as below:

[HouseRules]
defaultRuleset = "My Mod v2"
loadRulesetsFromConfig = true

It is also possible that you have a different cfg filename as that was incorrect on the HR website until a couple weeks ago so make sure it is named MelonPreferences.cfg and in the correct directory.

Closing this issue is we've not been able to reproduce.