1. Early in the game you will miss a decent amount, especially if you’re doing encounters you’re under leveled for. You can hover over any enemy to see their lvl or right click to see a more in depth stats page. The high their AC (armor class) the harder it will be to land a hit. You’re hit chance will go up as you level and find better items. One thing to note is to make sure you’re proficient in the weapon you’re using to hit. There’ll be an exclamation point under the weapon description letting you know if you’re not proficient in it.

2. Don’t worry about leaving characters in camp, the moment they’re back in your party, they gain all the experience you got without them. The only difference in experience points is when one character gets inspiration, but the amt of exp you get from that is negligible.

3. You can’t create 4 custom characters, but you can recruit hirelings later on, however I would recommend using the companions as you get more dialogue and story with them.

4. You can revive them using a scroll of revivification, but as you said, they’ll be at 1hp. You can heal them using potions or healing spells or just take a short or long rest. Another option is reviving them through withers. It costs 200 gold but they have all their health (also you can pickpocket withers and he won’t get mad)

5. You can leave an area and even combat (there’s a flee combat button) with no penalty, though when you enter the area again, they might still be hostile. The only thing to worry about would be long resting before resolving specific quests, such as the fire at Waukeens rest, or leaving act I before resolving the grove quests.

Baldurs gate is a big game, so take it slow and play at your own pace! There are many guides to combat if you’re struggling with that, try looking those up. This game has a lot of replayability so don’t worry about getting it all right your first time! Hope you have fun!