1) Cartridge bearings, they only wear themselves out whereas traditional style loosely packed balls in a cup wears out the cup
2) Replacement bearings - that depends on which headset you're talking about. Replacements are obviously available for most
3) Cups don't typically seize into frames like seatposts do. So you're worrying about nothing. Alloy posts in steel frames don't seize if it's properly installed with grease/anti-seize and maintained.