1) I'm not totally sure.
2) Sadly, I don't know of one.
3) Yes, it does.
4) Depends. Gallows are faster then footknights - and they don't carry all that armor, meaning they're far more useful in the desert. They also don't have anywhere near the building requirements that knights do - knights require horses, armorers, and a royal court. Gallows require just a swordsmith, and clansmen just a fort.
5) Disable, I don't think so. Though, I'm sure at least in VI you can probably move it to some ridiculously late date like 20,000 AD.