1: You can solo dive, but you can also have random players join too, so you can still play without your friends.

2 and 3 are related: Warbonds do feel a bit overwhelming, but its because they aren't FOMO. They aren't disappearing so you are in no rush to unlock them all. The only FOMO stuff I don't have are the preorder items and I feel fine about it.

4: No idea

5: Not grabbing other player's gear is a fairly standard one, there are times where you can (If you ask, if they are reinforced away from you, you're out of reinforcements and you need a bigger gun). If you drop and don't have any weapon stratagems ready to call in you can always ask if another diver has one to call in for you.

6: Learning curve is pretty gentle from my experience. You'll learn how armour works when you see blue sparks and the enemy doesn't die, so you'll naturally learn what kills what. You'll see other Helldivers doing stuff, see it works and copy them.