AAA XR-E Q5's?

How many are there?
Which is brightest for under $30?

All the high output lights in that price range are pretty crappy. If you raise your budget another $10-15, you get a lot more options with both quality AND brightness.

KD Buckle V4 Q5 running a 10440 best in its class for $23

There's the KD Buckle and there's the R2 AAA, both have pretty short runtimes even on rechargeable batteries. The KD one has better quality from what I've read.

The Liteflux LF2X also uses a Cree XR-E Q5. Would similar performance work too? The Rebel 100 and the Q4 bin are not that far from the Q5.

I got five KD V4. Their quality is very good. Threads are smooth but could be tighter but some teflon tape fixes the problem. Overall it's very good for it's price. I also had DX r2 (Aurora) buckle and comparing it to KD V4 it's very cheap made. Lots of solder to make positive battery contact. Body is thinner and was crushed a little so it was more elliptical than round. Threads are horrible and very thin. V4 is also brighter and has warmer tint. R2 advantage is that it's smaller than V4 and has more focused beam. Liteflux is whole another story - it has very complicated user interface with user-defined levels etc - opposed to one level V4 or R2. I don't have LF so I cannot share my opinion but you can find some reviews here on CPF. LF quality is very good and it's very versatile light capable of 40 hours runtime in lowest mode (intensity of about 1lm). IMO changing modes via twisty is bothersome and I personally prefer V4 + Fenix E01(for runtime) combo.

K.

I think the Fenix-LOD Cree-Q4-XRE is a very good quality AAA-light
with a bright output.

Although safer with Alkalines or NiMHs (Especially Eneloops), many
have said that 10440s can also be used.

I have mine on my keychain and the HAIII coating is proving to be
very tough. I can see no wear at all after deciding to put it on my
keychain about a month ago. Before that is was a shelf-queen.