Hey Morana! You did a great job there! :) I love opera, but I can hardly understand the lyrics when I hear it (and I'm Italian!!), so, really, kudos to you!!
About the "alla fin vi posero" part I think he actually says "alla fin riposerò". So yeah, your assumption is correct, even though you were not sure about it. :)
Where you wrote "E pensate si a fugire", it seems he says "E pensateci a fuggire".
In the "della? ... sorci indegni," part, I *think* he says "dell'aremo sorci indegni". That word doesn't exist in Italian, but that's the only one I came up with, comparing this text with the German one. It looks like an Italian version of "harem"...does it make sense to you?
The missing sentence is "nella rete voi cadrete".