[Oli]

1. Yes, compatible with Ninja 2.
2. It converts the Ninja's mounting system to Socket 478. You remove the Socket 775 bracket and you the 478 bracket instead which comes with Ninja 2.
3. This mounting method is more durable and safer than pushpins. Nothing will slip out or unlock accidentically.
4. Pushpins offer a lot of pressure so the gain on behalf of the retention kit is not very high. So the kit delivers at least as much pressure as the pushpin method but basically a much safer connection between mainboard and cooler.
5. It depends on the size of your fingers if you'd find it easier to remove or mount Ninja 2 with ret kit or w/o it. I personally do not like Intel's choice of using pushpins as reference design for Socket 775 and I sympathize with the old S478 method that ret kit uses - but I couldn't say which I'd find easier to handle. It highly depends on the mainboard components situated around the socket. As you already mentioned there's little space to operate the pushpins and I personally don't experience the ret kit method as much more handy.
6. The copper spacer is recommended for Socket 775 to ensure that MB components around the socket are cleared concerning height. Nevertheless it is possible to leave the spacer out - but remember that you'll have to replace the plastic washers that come with ret kit with thin ones made out of paper or cardboard - otherwise the pressure of the cooler's base plate onto the CPU heatspreader could be insufficient.

If it comes to any problems with parts of the ret kit's rails hitting MB components it is necessary to use the spacer although this has impact on the temps due to the additional isolation and thermal grease you already mentioned. I'd prefer usage w/o spacer if it is possible.

Cheers, Oli