You can still recruit the Ashen Wolves in Chapter 2 of the main game (provided you own Wave 3 DLC) – no need to play Cindered Shadows first, though the side story gives you their backstories.
No. All v1.1.0 saves are compatible. However, if you start a new game after installing the DLC, you’ll see new Abyss characters appear in Chapter 2. Fire Emblem Three Houses -NSP--Update 1.2.0- -2...
Without 1.2.0, you miss out on:
If you have the base game (v1.0.1 or 1.1.0), applying 1.2.0 is essential before starting a new playthrough, especially on Maddening difficulty where Trickster’s utility and Dark Flier’s mobility shine. You can still recruit the Ashen Wolves in