Well, the back story on all the quests and events in SWG is considered some level of 'canon', so apparently at some point around .5 ABY Leia and Wedge were operating out of a base deep in the mountains on Corellia northwest of Kor Vella. Not sure if it had a name, but it was quite a pain to get to on the ground and seemed kind of tiny in terms of what I'd expect for rebel bases. She had the droids with her.
Hmmm , Corellia is a core-world, and was under the control of the Empire during this time no ? no, not exactly. Corellia withdrew from the Republic prior to the Clone Wars and became a quasi-independent system. as such, it was not part of the conversion to the New Order. it was still basically subservient to the Emperor whenever push came to shove, but it wasn't integrated into the rest of the Empire on a day-to-day basis. it's also worth noting that, for most of his rule, Palpatine treaded very lightly in the Core Worlds. he wasn't nearly so obvious and overpowering with the use of force. this started to change with the dissolution of the Senate and the destruction of Alderaan, of course, but he was careful not to alienate the Core too much by being too heavy-handed.
I don't believe Derra IV was ever a Rebel base. It's noteworthy because of the Imperial attack on the Rebel supply convoy there. The convoy was carrying supplies for the new Hoth base and the Imp attack so succesful that Hoth never stood a chance when it was attacked shortly there after.
They were travelling at sublight speeds as the convoy passed through the system, which means that either they dropped out temporarily for some reason, or that the convoy originated there. In fact, Commander Narra's dialogue in the first episode seems to indicate that they had just taken off from a base nearby, and were almost immediately intercepted by the Imperial task force in the system.
A base nearby does not have to be a base in the same system. As for dropping out for some reason, sometimes it's better to plot a course through other systems to go from point a to point b because a straight line between the two points takes longer. According to the Astrogation Gazetter, Coruscant to Tatooine is 22 day and 14 hours. However, A trip from Coruscant to Corellia is 4 hours, the same for Tatooine to Corellia. So it is faster to plot a trip from Tatooine to Coruscant by going through the Corellian system.
Sullust had just publically announced its intention to join the Rebel Alliance. Few worlds ever did that (Mon Cal, Elom). I don't think that the Rebels had an actual base there, but they used a friendly planet as a massing point. Besides, Sullust is on the eastern half of the galaxy, just like Endor.
i always imagined that they didn't have a base at sullust... they just did what they said.. assembled the fleet there... perhaps they used some of the local orbitaldocking stations and whatnot, but i figured that they used a location that was sympathetic to amass the fleet while planning a strike that might win the war... more similar to simply pulling all the ships together in friendly waters rather than having them at a homeport... (for the US Navy, perhaps gathering the fleet just outside the persian gulf, and not like gathering them up in Norfolk or San Diego or Pearl Harbor). i always envisioned that prior to this, the rebels had kept their fleet scattered, to avoid taking losses, and to engage the imperials all over the galaxy, to make the imperials spread out. sullust might have marked one of the first times since the meetup at the rendesvous point after hoth that most of the rebel fleet was together in one area. so.. what were all those capital ships needed for at endor, according to the plan... just drawing the fire of the batteries on the DS2, until the smaller ships could fly in and blow it up? or were they supposed to fire on the DS2 and take it a piece at a time if the initial core assault failed... (or to bombard Endor to destroy the shield generator from orbit, Han's strike mission failed)
Weren't they supposed to engage the Imperial cap ships there? It can't be the one about drawing DS2 fire because they didn't think it was operational.
i think by not operational, they meant the big superlaser on the DSII, as well as probably the engines and hyperdrive... but im sure some of the surface turbolasers and other smaller defenses could be assumed to be online.
Plus, I'm sure they thought that they would encounter a few Star Destroyers guarding the Death Star, though not as many as there actually were. And Sullust wouldn't count anyway, because it was after Hoth.