I agree with others who found Jamie Alexander's(?) performance as Sif to be filled ith charisma and presence. She made the character's stilted speech and haughty attitude seem perfectly natural and not at all phony or ridiculous. I was kind of blown away by the performance, actually.
I felt the opposite. Her performance did seem a bit too stilted. For me, everything you said describes Elena Satine as Lorelei though.
I feel the same as
AR07, and am honestly really surprised that anyone was raving over Sif in this episode. I didn't think Sif displayed much personality whatsoever, other than having a stick up her ass. Her formal manner of speaking didn't seem at natural to me (Hemsworth pulls it off, IMO, Alexander does not). Lorelei was great, though, and I actually felt a lot of sympathy for her by the end of the episode, despite the fact that she's clearly on the wicked side.