Dark Lord of the Sith was his title given to him since he was a "master" of the dark side of the force.
But we never see any Dark Side practitioners in the OT other than Vader and Palpatine. Why would secular Imperials refer to him by a cult title? He was clearly exercising authority over them that did
not derive from his position within the Sith cult. Tarkin even mocked him as the last member of the Jedi religion. No, when he's addressed as "Lord Vader" it's clearly a title of aristocracy, not clergy.
Tarkin was actually a Grand Moff over the Moffs of many sectors. He held a higher power than any regional governor could imagine. I would imagine that Moff tarkin also held some extra power aside from that of a Grand Moff since he was the Chief Officer of the Death Star.
Tarkin was a Grand Moff and Regional Governor. The Grand Moff dignity might have indicated some extra-regional authority, but it does seem strange that he brags one moment that Coruscant has given control to regional governors then the next blows up some other regional governor's planet.
Echo Base was the rebellions headquarters. It is where many Rebel leaders met and discussed tactics. Basically imagine it being like the only talked about, but never seen War Room of the United States, which supposedly sits under the White House. Luke Skywalker recommended the location of the icy planet of Hoth to the Rebel leaders because it was so remote, had little resources the Empire would want, and was simply a difficult planet for the Empire to search. Remember how Echo Base was almost quickly dismissed by Imperial officers before Vader walked up and declared it to be the base. If Vader had never seen the image, maybe the Battle of Hoth would never have taken place.
Why would anyone in
the Rebel headquarters be discussing tactics? As Patton said, no one above battalion level should be discussing tactics. Definitely not echelons above God. Why would they even need to meet in one place to discuss tactics anything? Vader teleconferenced with his commands; he only met them in person when he wanted to kill someone, and sometimes not even then. The Jedi Council teleconferenced in the prequels. Why didn't the Rebels work this way? What is this "War Room of the United States"? If you mean the White House Situation Room, there's a good description of its workings
here. Why didn't they meet on a ship, like Mon Mothma had the brains to do in RotJ?
Hoth was hard on their equipment, and hard on their reconnaissance -- they had to ride pack animals around just to see what was happening right outside their base. It wasn't defensible, because those huge plains practically invited an armored assault, and putting their power generator above ground where anyone can shoot it wasn't too bright. Freezing all the time and frequent cases of frostbite, snow blindness, and hypothermia couldn't have been good for morale.
Echo is probably just the successor to the Yavin cell -- a few X-wings and an Alderaanian tart with money.
I would imagine that Han and Lando could be given Field Officer Commissions much like the United States Military can during times of war.
They could be commissioned Admiral of the Ocean Sea, but
why? Field commissions, and other direct commissions, or only granted when the potential officer has shown the capability to perform in the grade to which he or she is being appointed. When did Han Solo or Lando Calrissian ever prove themselves capable of serving as senior military executives? Han couldn't even manage his
own finances, let alone a field army's logistics.
They did re-enforce the bunker with a whole legion of Stormtroopers, plus they thought they were only going to be dealing with a group of saboteurs, not a bunch of hairy mokney things with super powerful stones throwing muscles.