Wow, what a twisty-turny thread this has become. Still, lots of good stuff here.
Before I launch my long post, I really like the color of the brownish, background you used with the orange cat. I think it looks very elegant and could be great with no props.
Although the whole "melodrama" thing may have been a bit blunt, Julie is right that you will get better results quicker by concentrating on what you have going now rather than adding the complication of a back light. Yes, it can produce nicer photos, but so can mastering what you have first. In any case, you've come a long way.
I did just over 600 rescue portraits last year and going back to when I first started them in 07 and comparing them over time, there is a HUGE progression. For those portraits, I only added on piece of equipment which was a second flash.
I only recently started using any sort of back lighting and quite frankly in my situation (all on location rather than set up in the same place all the time) the back light was a pain to integrate. I've got it now, but I think you'd be better off waiting a while.
As for the ABC's course, you'll find that many of those type courses are about the same. I'd venture to say that you would learn more that would help you with your cat photos from Moose Peterson's Wildlife Photographers' Base Camp although the subject matter is completely (almost) unrelated to the cats you work with. That said, time constraints probably would keep you from it since it is a week long course away from the cats. There are other good courses, out there, but you have to vet them carefully before hand.
Moose would agree that the histogram is worthless. His philosophy is basically; "The HG will tell you that there is a problem, but wont tell you where that problem is." Moose gives a little more credit to blinkies (highlight warnings).
Artie Morris, on the other hand, lives by the histogram.
If the top two wildlife photographers in the country can't agree on the HG, why should anyone else?
I fall somewhere in the middle in that I'll look at it, but the HG is the last thing I'll check on the back of the camera. Blinkies are the first.
The funny thing is that a couple of us in my area are considering pet photography workshops that would use rescue animals as "models." The rescues would get good photos plus a portion of course tuition. That's far off though.
Tripod? No way! At least not for cats & dogs. Testing and learning? Sure. Cats & dogs require the flexibility of hand holding. I roll around all over the floor at my shoots to get the angles I want, which pretty much eliminates tethered shooting as a possibility too.
As for white balance, a piece of cardboard or white paper is not going to be as accurate as the expo disc, but it will get you "close enough" a lot less expensively.
Personally, I don't bother with custom WB in camera. I would if I shot JPG only, but I shoot raw. It is easier, faster and more convenient to do a mass WB adjustment on the computer.
To accomplish this, I use a Whibal card when I'm not shooting on a solid white, black or grey BG. http://www.rawworkflow.com/whibal/
It is one click and sync for me which takes all of 5-10 seconds.