Is Sephiroth's sword practical?

It's absolutely useless. Its closest equivalent is the N/Odachi, an exceptionally long Japanese sword that was used as an anti-cavalry sword (you kill horses with it).

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Consequently, can a sword cut a skull?

Yes, provided the cut is done with proper technique. Pretty much any cut centric sword can. It can also glance off as the skull is rather deflective, but connect properly and yes, it can. Can a katana cut your hand off? Yes, a real Katana can cut the person in one hit. This sword is famous for its sharpness and flexibility.

Correspondingly, can a sword cut a bullet?

Theoretically, yes. Bullets have been fired at swords, and yes, they were cut in half or shattered. Problem is they don't deflect much and now you have multiple bullet fragments hitting you, likely doing more harm. And another question, can a katana break a longsword? Katanas have a thick spine making them a sharpened steel club. Longswords were made of spring steel and were longer and thinner. If both were made well and new neither would break.

Why are Japanese swords folded?

The process of folding metal to improve strength and remove impurities is frequently attributed to specific Japanese smiths in legends. The folding removes impurities and helps even out the carbon content, while the alternating layers combine hardness with ductility to greatly enhance the toughness.