Hello,
Peter ...
You ask, "Is there a "Guide to the Kingdoms and Clans of Playmobil" of sorts?"
Question pertains to the Knights realm...
I'd suppose it might be possible to start a Wiki of sorts on playmofriends? Any suggestions?
In answer to your question about a "Guide" ...
"NO!"(Zirndorf couldn't even develop a simple set numbering system!) ...
In answer to your proposal of starting a "Wiki" here at Playmofriends ... "
YES! It might be a lot of fun to create some order out of Playmobil's chaos." ...

In looking at your example:
(questions in blue)- dress code (colour, armour, attack troops, castle defenders)
Would you include the original chrome plate helmets, chrome torsos and chrome breast plates as well as the orange shields?- special weapons and machines (Eagles and Green Dragons have siege towers, Black Lions have a catapult unit)
What about the old Nurnberg Guard cannon, the Red Dragon cannon and the Blue Lion cannon?- evolution of clans (the 'new' Red Dragons being related to the 'old' Gold Dragons)
This might be a real stretch. Might also require some story-telling.- alliances (Swan Knights with the 'old' Kings, how the Unicorns fit in)
Might require some more story-telling and some blending of historical and fantasy!- independent clans/knights (eg, it looks like 3315 Evil Knight is either for hire (to the Lions) or just decided to join them for now?, then there's the Merry Men)
Perhaps some very special "hero" figures as well?- list of 'historical' battles
Garden Wargaming? ...

- rise and fall of empires (time-line)
Hmmm ...

All the best,
Richard