1- This is all made from the grown up. You could say it´s an alternative take for Equestria to fit the mature themes of my story. That are moving along from trauma, self aceptance and may more things.
2- The House of the Sun is Indeed an religious organization, that sspins arround the Queen Celestia (just as Lauren Faust envisioned). They are mostly allies and friends, but some of the higher ranks of the organization have their fair share of skeletons on their wardrobes.
3- No, They are a cult surrounding Nightmare Moon, that thinks that Celestia was evil and a jealous of her younger and more sweet younger sister. Obviously they were misleaded, and is the job of Luna as Princess to convince this cult that everything waws a big misunderstoodment (And also, The Children of the NIght originated as a griup of orphan ponies who Luna took care of when she was young)
4- Yes and no. Yes because they act on their own and are as old as Canterlot, but no because of Shining Armor being his captain, and because the two faction tends to work as one.
5- They have trains, Zeppelins and Air Balloons, and even radios and TV´s. But the guns are restrainted to the Southwest section of Equestria due to the Companies War. So the main factories of pistols and rifles by royal decreet are restraingted to sell their products only ont their zone. As for the Gun Slinger Syndicate is operated by a group of three stallions that run the syndicate in order to find some jobs to the other Gun Slingers. Some Bounties, hitman jobs, etc.
6- The basiucally are a Elite Group on Cloudsdale that is on charge of go and hunt every big monster that represents a threat to Equestria. Dragons, Constelation creatures, Sea Monsters, etc.
7- They are in charge of the general Sea Trading. Delivering all kind of resources arround Equestria and even some from foreign lands. Facing all the dangers that represents those Journes. Storms, Sea Monsters, the Hypogryfs, Viking Assaults, etc.
8- They are basically a bunch of war mongering demons from Tartarus that Tirek used to lead. But since he is gone, all of them devolved to mindless beasts capable of destroying mountains by ther most powerfull pounches.