1: Yes, but it is very difficult
2: Lots of milling operations and setups required. Expert machinists have done it, an advanced home machinist might be capable but if you don't know what you're doing forget it.
3: The Century castings are not finished enough to be considered "inch or metric". The main difference is the magwell cutout for the lip on the front of the mag is larger on the inch. The ones on my castings are too small even for the metric. The rest is mainly cosmetics on the exterior.
4: Yes, the receiver takes the full force of the lockup system, so at least the section between the breech and locking shoulder will need to be heat treated.
5: Nope, there are some incomplete and/or incorrect prints some people have reverse engineered from commercial receivers, but nothing "official". Part of the reason it is so difficult to complete one.
Check over on the FALFiles, there are lots of posts on this subject. Many people have modifed the design slightly and used them for pistol caliber conversions as the blowback type system puts much less stress on the receiver and some of the internal cuts can be deleted with a different bolt system.