Best Of

Take a look at this, you also could go fourther and use a simple compression approach here
here

No.
The idea is to not trust webhooks.

Disclaimer: you don't have to follow everything I say but only what works for you and you can start simple then add changes incrementally later on.
I might sound paranoid.

When PathCreate is called and validated (and allowed via ResultCode: 0):
you try to add the GameId to a list of open games. (e.g. SharedGroupData)
if such GameId exists for the same enviornement (Region, AppId, AppVersion, etc.) return ResultCode != 0.
you add the UserId / ActorNr pair to the list of actors joined to the game.
When PathJoin is called and validated (args checks & AuthCookie) you add the UserId / ActorNr pair to the list of actors joined to the game.
When PathLeave is called and validated (args checks & AuthCookie):
if ActorNr == -1, ignore/skip, this means the actor never joined successfully.
if IsInactive == true, mark the actor as inactive in the previously saved entry in PathJoin.
if IsInactive == false, remove the actor from the list.
When PathClose is called:
try to find that GameId in that list.
count the active actors (it should be zero).
if Type == "Close" remove the entry
if Type == "Save", save the State and optionally remove the GameId entry and actors list.

Hey @Only4Gamers, When a Master Client leaves and there is at least another actor joined, a new Master Client is chosen by the server, so OnMasterClientSwitched is triggered on other joined actors if any.