... placing them effectively at the stage corners, pointed towards the drums (in the center).
Any thoughts on other uses of my available mics?
My thought... and it's just a thought, there are no black and white, right or wrong answers...
Putting your mics far apart like that puts a lot of "phasing" between the mics, because it takes time for the sound to travel through air (about .001 seconds per foot). If the guitar cab is on the right, the right mic is closer. But the left mic gets some guitar too, but it hits the left mic .010 seconds after the right mic. Especially if you listen with headphones, it can create an odd effect.
An alternate approach would be to put 2 mics close together, in the front center (perhaps even clamped onto the center singers mic stand) and have those mics pointing towards the back corners of the stage, at 45 degrees (such that the included angle between 2 mics is about a 90 degree right angle). Then the capsules (business end of the mic) should probably be about 8" apart. What I have just described is what we call a "DIN" configuration. Some people swear by XY setup whereby the capsules are stacked on top of each other so there is no timing issues, others like the DIN I've just described, others like ORTF which is about 7" apart, and 110 degrees instead of 90. There are pictures here somewhere. The point is that having the mics even a few inches apart makes a difference in how it sounds with regard to this timing/phasing thing. Sometimes people split the mics 2-3 feet apart, and some people like it, but it's not my favorite.
Your brain is wired to get a feel for "where things are" in space based on the fact that your ears are about 7 inches apart, and that's were the whole "set the mics 7-8 inches apart" comes from. If you use that to your advantage it can help things sound more natural, especially with headphones.
Sometimes people break the rules and it works great. Somewhere I have an Allman Brothers tape with 15 foot split omnis that sounds great. I tried to do that one time and I hated it.