ULN2803A PWM LED High Watt/Ampere
I can only answer part of your questions.

1) I simply don't know how much current a Raspberry pin will put out, and Broadcom is very short on information. I recommend this: buy one 2803, hook it up with all the inputs tied together but no load, and measure the voltage at the inputs.

2) I can guarantee that with all the inputs tied together, you will draw less than 12 mA with an input of 3.3 volts. How can I say this? The spec you listed gives an input of 3.85 volts, not 3.3 volts. Lower voltage will give you lower current. But measure it. If the Raspberry cannot provide close to 3.3 volts when driving all 8 inputs, you may have extra problems with point 3.

3) Yes, you can pull 2.5 amps with one chip. I don't recommend it, but you can do it. If you do, you MUST provide a decent heat sink for the 2803. If you just use a bare IC you will burn it out. You should assume something like 1.3 volts across the 2803, and at 2.5 amps, your total power will be about 3.3 watts. The thermal resistance to ambient for this chip (section 7.4 in the data sheet) is 74 degrees per watt, so the heart of the chip will reach (3.3 x 74) + 20 degrees, or about 260 degrees C. Per the data sheet, the maximum is 125 C.

If your input voltage is low, the outputs will not turn on as hard, and the voltage drop across the outputs may be greater. In that case, the power dissipation of the chip will be higher, and you will need a bigger heat sink. But this is something that you will need to experiment with to find out. I suspect you'll be OK, but I'm not giving any guarantees.