Before anyone asks, rounded panels with faded borders are for mid-comic fantasies or flashbacks and are as such to differentiate those panels from others. This doesn't apply to dream sequences.
(Of course, I'm totally making up these style rules as I go, so it's entirely possible there's an example in the archives of me breaking it. Way to go, past me!)
Susan is dressed in an outfit reminiscent of the original Star Trek uniforms, while Diane is dressed in an outfit reminiscent of the mirror universe version of said uniform. This doesn't so much mean Elliot assumes Diane is evil as it is reflective (HA!) of him meeting Diane second.
The basic execution of the Spartacus reference joke is one that I know with certainty has been done before by many people, but I couldn't help myself. It was just so natural to the moment that trying to write something else felt like forcing a car to suddenly change direction while driving at high speed. I don't think I could have done it without flipping the whole dang comic over.
Speaking of Grace, I can think of at least one reason she'd be showing up in Elliot's dreams on this particular in-comic night.