If it’s not a narrow wide chain ring you may drop your chain so you’ll need a chain guide up front to help generally keep the chain on especially on the rough stuff.
Internal width of 9-10-11 speed chains should all be 3/32nds so should work. It’s the outside width that changes to allow more sprockets on the rear
42 or 44 depends on so many variables - tyre size, terrain, style of riding. If you have a low cadence and like grinding a big gear go for the bigger ring, if you like spinning go for the little one. There won’t be a huge difference anyway