I don't think there's a great answer sadly: all the options have downsides.
Rex crank 1.1: Claimed weight 489g without chainring or BB... also says chainline is 56mm which seems crazy wide (SRAM is 49mm) and might affect shifting if it's correct.
Sram spider: not clear if it's going to give proper chainline with the GXP crank, or wider (designed for BB30)... and given the stated chainline above... wider might really be too wide. Weight of 175mm GXP with XX1 spider (no ring or BB) is 520g (based on one I had) and I'd expect the QX1 spider to be similar to the XX1 in terms of weight. So that's a little heavier than the Rex option.
Specialized spider: Sworks crank has 30mm spindle and won't work on BB92 without you getting a custom axle Hirth spacer to widen it a little bit (hard to find and possibly expensive). It's a bit of a hack to get it to work, and again the chainline would be around 2mm wider than intended by Rotor because of the axle spacer, but this would definitely be the lightest option. I think you'd be looking at around 415g. You could also use the spider on a Lightning crank, but some people don't like the way those look, they're expensive, and the chainline would again be a little wider than Rotor
Other options would all require you to get a custom spider made. This is actually a route that I'm pursuing although my BB is different (73mm BSA):
Hollowgram SiSL/SiSL2: needs lipless spindle (available from R2-bike), plus custom spider (trying to get Experimental prototype to make one for me)
RaceFace Cinch: needs custom spider, but otherwise would work as is. But availability on these is still somewhat lacking, and since the system is so new, getting a custom spider made for it is going to be harder (Hollowgram has been around long enough to make it easier)
Either of the above is going to be pretty competitive with the Specialized weight (think they'll all be within 10g of each other). They're a hassle, but you could make sure the chainline is what you want.
Pretty much any really light weight setup is going to use a 30mm spindle which I know you said you don't want to do. But maybe it's something you should reconsider in conjunction with a new Race Face BB (should work with any
30mm crank with appropriate spacers I should think)... I read this comment on a pinkbike review
of the cranks that addresses your concerns:
"Since the Next SL cranks have a 30mm spindle, in order to make them compatible with bikes that use Press Fit 92 bottom brackets a bearing with an outer lip is pressed directly into the frame. Eliminating the retaining cups allowed Race Face to use the same size bearings as are found in a 24mm spindled set up."
In that case the easiest light setup is Lightning crank (with Specialized QX1 spider) and RaceFace BB. Next is the Rotor crank itself. Anything lighter than Rotor (other than Lightning) requires you to pursue a custom spider.