i don't think comparing vader's duel(s) with luke to the fight with the rebels is in any way reasonable. vader wanted to turn luke, he wanted to kill ezra and kanan. he definitely would have killed ezra if kanan had not interfered. quickly and without any "toying". he didn't need or want anything from them, his plan would have worked just as well with them dead. it was obvious that they were not strong enough in the force to pose any danger to him, so learning anything about their abilities seems like a waste of time to me. the duel between vader and kanan seemed totally out of place and unnecessary.
I cannot see why you think this. Kanan is almost a Jedi and has a lightsabre, an obvious threat appears that it looks like he is best suited to deal with and so he attacks. Why you think Kanan would not try to protect himself and the rest of the Ghost crew, I have no idea. Vader wanted the Rebels to escape overall but seemed to want to kill at least Ezra; but he was prevented by Kanan. I don't see anything contrived in that.
ahsoka didn't join the ghost crew because the ship she was on was close to being destroyed. the ghost went out because phoenix squadron was outmatched. ahsoka coming along was obviously for plot reasons. there was nothing for her to do on the ghost. it would have been easy to include a scene where she sensed something about the tie-pilot. then coming along would have made sense.
The Ghost going out was their best chance of defending the main ship. Ahsoka is a very experienced pilot and fighter, why should she not go along to help? If you have this idea that she can't help because the pilots chair was filled and each gunnery seat was filled then why did Sabine or Chopper or Kanan go along? No-one knows exactly how things are going to play out - whether someone will get hurt, they'll need to repair the hyperdrive, whether she'll need to step in and do the shooting because she's the only one who can hit the Ace fighter pilot attacking them if Zeb proves unequal to the task or if she'll suggest a good tactic or idea to Hera or simply to keep an eye on them. There's nothing useful she can do on the main ship to stop the attacker. There're plenty of scenarios where she can help if she's on the Ghost, and she was right.
noone can tell me with a straight face that a single tie (even vader's personal ship) can attack several cruisers and a battleship and expect to live. that was complete crap. if i remember correctly the cruisers didn't even shoot at him, while he didn't use torpedos or other weapons one might use to attack such a big ship.
It's truly absurd. Like an X-Wing destroying a Death Star. Point is that Anakin Skywalker was the best pilot in the Galaxy. No-one was better than him according to people who had a lot of experience with these things. He took down the Malevolence in TCW, and that spider-general's capitol ship in a later season. He has form with this. It's something of a family trait.

