Yes, all runewords that existed in the original Diablo 2, can be made in Diablo 2 Resurrected
There are a total of 78 runewords in Diablo 2
Yes, runewords can be made in any non-magic (magic, rare, crafted, unique, set) item with the correct amount of sockets.
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.
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-only runewords were used as a reward for players that played in ladder. The sole reason for ladder-only runewords was the incentivise ladder play.
In Diablo 2 Resurrected, there is no difference between ladder and non-ladder, so also in single player, all runewords can be created. In the original Diablo 2, ladder runewords are restricted to ladder multiplayer only.
In Diablo 2 Resurrected, there is no difference between ladder and non-ladder. In the original Diablo 2, 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.