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The exposure is a balancing act, and you really want to use the flash as fill light only unless you want to end up with black backgrounds (which can be effective sometimes, don't get me wrong).
My first comment would be that using Av mode on the camera and ETTL for the flash, you really are relying on the camera to guess what exposure you are after. You don't know what you're going to get. Considering this, it shouldn't surprise you that you're getting overexposed subjects.
Av mode, you choose the aperture and ISO but the camera chooses shutter speed. What if the speed it chooses is too slow and you end up with motion blur? Yes, the flash will freeze motion to a certain extent, but not so much if you're only using it as fill. Moreso if it's the only source of light and you want black BG.
Can't recommend enough that you use full Manual mode for camera exposure. Take control of your exposures, especially if it's going to be an income for you. You'll end up with more "keepers", because you are dictating how you want the image to end up, not letting the camera guess it for you.
Yes, you can successfully use Av mode, but IMO it just throws in a variable that I don't want.
What I do is set camera to M mode, dial in what aperture I want (usually f/11 for up to 1:1 magnification macros), set a shutter speed around 1/200, then check the exposure and set the ISO to give me around the correct exposure. You don't need to have the subject all lined up and ready to shoot to do this, just point at what the background will be. You don't even need to be focused. And you don't need to have the exposure spot on, you might leave it underexposed by a stop or so. This will allow for some variation in ambient light, allow the subject to be better lit than the background, yet still not have that black background look. And it makes the flash not work as hard, so less power, so quicker recycyling and longer battery life, and less chance of harsh light.
I do limit the ISO though. You don't want it to be too high to control noise. Typically ISO 400, maybe 800, no higher. If you're getting up to 800, you can always drop the shutter speed back down to 1/100 instead.
So now your camera is set, time for flash. I've started using full manual flash lately as well, but might be easier for you to get used to ETTL first. If you have FEC set to zero, and the camera exposure set to underexpose by around one stop, the flash might expose the subject correctly depending on what metering mode you've used. Maybe try centre-weighted if your subject in in the middle. It may be slightly overexposed, so you just dial down the FEC to say -2/3 usually does it. It's a bit of trial and error, and every shot will be different. But you get the idea.
Finally - diffusion. If you're using ETTL flash, adding a diffuser won't change any of the above process because the flash will compensate and light loss that the diffuser takes from you. But the bare MT-24EX flash can be pretty harsh, so I do recommend doing something about diffusing it, and I've previously posted links for you on how to do that, and also about macro flash photography as well.
If there's enough ambient light around, do you even need flash? Personal taste. Probably not, but personally I always use if for fill. Controls the contrast a bit better, not as harsh. You can take out bright sunny highlights that you often get off shiny insects.
Opening up aperture? I wouldn't. Even at f/11 your depth of field is razor thin, opening it up only makes it thinner. Use the ISO, to a point as mentioned above.
Sorry about the long read. Hope it all makes sense.
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