1. I run Seagate ST3000DM001 in my DS. However they are listed in the compatibility list. I would bet they will work just fine but probably just not tested by Synology so they aren't on their list.
2. The DS has UPS monitoring built in. You just connect the UPS to the DS via USB cable and then enable UPS support and it will shutdown the DS before the battery runs out. I'm not sure how long the DS waits to shutdown as I haven't had a long enough power outage and haven't really seen it documented anyplace but its suppose to shutdown automatically. I have a 2200Va UPS supporting my DS1512, 2 small servers, a router, and a switch and the UPS shows it will hold the equipment for 3+ hours so it will be very rare to ever shutdown.
3. I don't have an ipad yet but planning to get one. Your suppose to be able to print from the ipad through the DS to any printer that the DS will support. At least that's how I read it.
Remember: RAID is not a replacement for backups
DS1512+, DSM4.1-2647, 3-3TB Seagate ST3000DM001