1. There isn't enough headroom in a 5 gal carboy for fermentation of 5 gal. You'll lose a lot of active yeast and head-retaining protein out the blowoff tube. It's possible I suppose but not ideal.
2. I haven't had experience adding water after the fermentation. I don't see that it would be too much different from topping up to volume after the boil as long as it's boiled and cooled. That's probably the biggest problem - infection possibilities.
3. When transferring to secondary never pour. You want to have as little contact with air as possible to prevent oxidation. Many people purge their secondary with CO2 to minimize this even more than just racking.
Edit: If you want to strain when racking to secondary, you can use a vinyl mesh bag on the end of the hose, just keep it all submerged to reduce oxygenation.
Primary: 20 min IPA (14oz cascade/5gal), partigyle pale
Bottled: Orange Cream Ale, Big Red Ale, RIS, American Stout, Nut Brown Ale, American Amber Ale, Brown Porter