#3
Megachords had really useful piano options but they underwent some changes and now they don't work. Maybe they'll fix it?
#4
There's no really a site like this dedicaed for piano. I have seen a lot of Guitar Pro tabs with the keyboards/piano splitted, one track for the right hand and another for the left, and they focus to be played on the guitar. If you know how to read sheet music this method is not useful. I started to transcribe songs in Guitar Pro using a simple track with the G and F clef in the same page. 

BTW, with MuseScore you can open Guitar Pro files and it's open source.

Good luck!