It's 512, but 512-351 reserved for system. You can check any android phone, you won't get full 512/1GB/... sure got part of it reserved for system.
PC 512MB get 504? Quite impossible lea. PC should get full. Not full only if you shared memory for graphics.
On Data(connected, less transfer) it should lasts a day. Around 3% an hour on standby. If keep on with data transfer then battery drop like water flow...

ya, replaced because stock ROM uses TouchWiz. think u can find it in the zip in my first post in XDA.

i m using the CiB calender, can sync with google, in fact, this is like the AOSP calender from google.