Your answers are a little wrong. I’ll provide some corrections.
Are RNG based item generators allowed if it is paid with freemium in-game currency? (Like Murder Mystery 2?)
No. The currency must be 100% free. The ArePaidRandomItemsRestricted policy determines if a user can interact with random item generators where Robux is involved: an 100% free currency does not violate the policy. However, to stay on the safe side with country laws, I recommend completely disabling item generators if it returns false. Roblox’s PolicyService may be about Robux-based random item generators, but a country’s laws may wholly outlaw random item generators.
Does paid item trading only include items that are paid only, or does that include something like Murder Mystery 2’s trading system?
Correct. If the item was bought using Robux directly or indirectly, it cannot be traded. If the item was earned or is 100% free and it is completely impossible to use Robux to get it, it can be traded. Again, to stay on the safe side, it’s recommended you completely disable trading anyway if this returns false, regardless of if items are free or paid with Robux.
If I mention something like discord in my game does that fall under the external link reference policy?
Discord is blanket banned in all games, PolicyService has not lifted this restriction as of yet. PolicyService also takes into account a user’s information (not just country, age as well) when determining policies: Discord would not be there for users under 13. Despite this, you cannot have Discord directly or indirectly present in-game at all at the moment.
Are the external links allowed for a certain age group or is it just a region, or even both?
Policies are generated by region, age and platform. It isn’t just region. This information is available on the API reference page for GetPolicyInfoForPlayerAsync.
Can I get a basic rundown on the China policies?
You answered this yourself by searching first.