No backlight and boot looping will be separate problems. For your backlight problem assuming it was working before the repair I would first rule out the screen and make sure none of the pins in the logic board connector have been pushed into the plastic. As the backlight components are on the other side of the board I doubt they have become faulty unless you connected the screen while the battery was connected. There are three pins for the backlight on the connector. You can also check them with a multimeter to check for a short or open line.
The boot looping will probably be caused by the battery or the connector, again make sure the pins on the battery connector are not pushed into the plastic. You can also put some pressure on the connector and see if it boots. You will also need to rule out a faulty battery.
[image|2561436]