I kind of assumed Lena lived in an old abandoned place of Magica de Spell's until she was adopted by Violet's family. Magica had to house her somewhere when she was just a shadow and Magica seems like the person who would have several places to live in. After Magica was defeated, Lena just continued to stay there until she was adopted. Either that or Violet's family adopted Lena right after she came back from the shadow realm.
I am absolutely no expert in this matter, but any mother/surrogate was probably a friend of the fathers. Also, Violet looks like her dads, so I wouldn't be surprised that she's biologically related to one of them.
I think the episode "Beaks in a Shell" did a good job showing a bit of Gandra's backstory, at least the important parts. I don't know if it's really important to know who her parents were. They're not really in her life and I don't know if they had a huge impact on her.
I think Scrooge will continue to adventure until he dies. He lives for it! Now, I can see him eventually passing his business duties to Webby and the nephews when they're older. But he will always want to adventure, especially with his family.
Considering how long Goldie has been in Scrooge's life, I think she has to be.
I don't think so. At the end of the finale, the villains are shown to be against Bradford's tactics even though he wants to end Scrooge and his family. I also think that Glomgold, Ma Beagle, Beaks, and Magica are very chaotic villains and Bradford has shown to really not like chaos, so I don't think he would work well with them.
I think the series as a whole was incredible. It continued to have great stories and really develop the characters. I do wish Huey had a little more development like his brothers and Webby had (though I loved seeing his friendship with B.O.Y.D and the Duke of Making a Mess)