That's a reasonable request - and should be submitted to Apple as an Enhancement Request rather than making a big public stink about how badly Apple is treating them.
Apple does listen to developers and is happy to hear ideas for improvements. If it really can be done without too much difficulty and doesn't cause any down side, then Apple is likely to adopt it. OTOH, if it causes negative consequences, they might not. But the developer is far more likely to find out by contacting Apple and filling out an ER than via a public complaint.
Possibly, but I'm not sure you can tell if your app is pirated.
Apple's policies allow for 5 devices (or is it 10?) to download the same app from one purchase. Each of them has a unique GameCenter ID. Telling which ones are purchased isn't that trivial.
So send in an enhancement request.
You don't. You're allowed to buy it once and use it on multiple devices. Which makes the "there are xxx users but only YYY people purchased my app" complaints somewhat disingenuous.