Pagz's solution to the QG/Yoda problem is perfect. I hadn't heard of that, but it's fantastic.
As for the first problem you list, it's only a problem if your goal is making sure the viewer doesn't know Ben is lying. If your goal is preserving the surprise at the end of ESB, then just having Ani die and Vader appear as a new henchment solves the problem entirely. My stance is that it's fine if the viewer knows Ben is lying but doesn't know how much he's lying - Ben is trying to manipulate Luke into wanting to take out the Emperor's new henchman while protecting himself. After all, it would ruin a lot of his take-down-the-Emperor plans if Luke knows Obi-Wan killed his father. And why would Luke listen to a guy who killed his father? Thus, the viewer thinks, the lie.
It isn't until the end of ESB that the lie makes even more sense.