Do you know if the cracks are GXP or DUB? It doesn’t really make a huge difference, except that you’ll need to change BB if you change axle standards. For 2018, it’s probably GXP which is SRAM’s slightly older standard. You can get replacement GXP or DUB cranks.
There’s no difference between 11 speed and 12 speed cranks at all. If they’re boost, the chainrings are different (compared to non boost) but you could swap the boost onto a new (SRAM) crank anyway, you if you found a pair of ‘non boost’ SRAM cranks that would work.
Re bearing covers: I’d ask in a shop, they may have some kicking around.
The splines in the crank arm were stripped out?
Sounds like it’s been suffering from a lack of maintenance for quite a while!
Nope. Riding along with a loose bolt will almost always damage the splines when the crank falls off, it only takes one time. Obviously SRAM have a stupidly high torque requirement.