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I have only seen 19x10, 19x11, and 19x12 (in 19s, intended as rears) in the reproduction C6 Z06 wheels. That doesn't mean they don't exist. Make sure you're not being offered a 19x9.5, that would measure 10.5" overall, outside rim to outside rim.
I just installed some 19x11 C6 Z06 repros, on the back of my 2001 Z06. The rims fit fine, but I went with wide 325/30-19 tires (almost 13" wide on the 11" rim). I had to remove the Z06 brake duct coming through the wheel well, and trimmed the inner wheelwell plastic above the A-arm, just to be safe. I forgot my offset, so I will have to check on that, but they are almost perfect for the C5, as they only stick out slightly further than the C5 Z06 wheel. If, in fact, you are getting a 10.5" wide rim for a C5 application, you should be able to fit them with a 285, 295, or 305 tire. Worst case scenario would be removing the inner brake duct, which takes one 7mm screw and 10 seconds of twisting and yanking it out.
Yes, your offset/backspacing would help.
I believe lowering the rear of the car on the stock bolts will help move the wheel/tire out slightly.