That's looking at it from Walt's paranoid perspective and after the fact. Gus taking it is the last thing in Jesse's mind. No way Walt even thinks that Jessie would blame Gus before he blamed him.
I agree on your second point.
Well let's go by process of elimination then.
If the kid didn't take it, there is only 3 places he could've "lost" it. Home, the lab or Saul's. That's the only 3 places he was all day.
He assumes Saul's, because let's go with what you said. Why assume Gus?
Walt convinces him Gus knows about the ricin....after all he did make it in Gus's lab, and Gus has cameras everywhere. Also, as Walt points out, they could easily go through Jesse's shyt when he's cooking. They team up, and take down Gus.
Then Walt plants the fake ricin in his house.
Fred.