Can runewords be made in superior items? Yes, runewords can be made in any non-magic (magic, rare, crafted, unique, set) item with the correct amount of sockets.
Can runewords be upgraded? No, runewords can not be upgraded. You can only upgrade a low quality base to a normal quality base, and then add sockets to it. The recipe to upgrade a low quality weapon is 1 Eld Rune + 1 Chipped Gem + Low Quality Weapon. The recipe to upgrade a low quality armor is 1 El Rune + 1 Chipped Gem + Low Quality Armor.
Why are ethereal base items so wanted? Ethereal weapons will have +50% base damage, ethereal armor will have +50% base defense. This is a huge boost, but comes at a cost of being unrepairable. However, when a mercenary uses an item, it will not use durability and is technically indestructible. Secondly, there are runewords that have attributes like Indestructible or Self Repair (f.e. Exile or Breath of the Dying), these runewords are also best in an Ethereal base. On top of the defense or damage boost, Ethereal items also reduce Strength and Dexterity requirements by 10.
Ladder
Why are some runewords ladder only? Ladder-only runewords are used as a reward for players that played in ladder. The sole reason for ladder-only runewords was the incentivise ladder play.
Can you make ladder runewords in single player? In Diablo 2 Resurrected, all runewords can be created in single player. In the original Diablo 2, ladder runewords are restricted to ladder multiplayer only.
How can you get ladder runewords in non-ladder? In Diablo 2 Resurrected the only way to get ladder-only runewords in non-ladder is to create the runeword with a ladder character, and wait for the ladder season to end. When the ladder season ends, all ladder characters are converted to non-ladder and all its item will be available on non-ladder.