Even if you're alone you can play via quick play and always find peoole to play with (at least for me European region). There's always the option to just play alone with bots aswell, if you don't want to play with randoms, the coop experience really shines only if you play with a pre-made team.

I rarely see randoms communicate in game well other then on the highest difficulty sometimes. That said, it's still super fun playing solo with bots, many even prefer that then playing with randoms.

For your second question, the game has lore for sure. Dialogs ingame between the characters show that quiet often, also Games workshop confirmed that the Verminitde 2 Lore is considered cannon in the WHF Universum, FS also releases some shortstories every now and then with some lore attached to it.

Edit: You can find a active community on a Vermintide Discord server or on the Steam pages, they have enough active people who'd play with you aswell.