Yes attacks go through walls.
Running on mount is visual bug.
toons bundled together doing stuff are bots.
Play what you find to be fun is my advice.

Welcome

I would start over. But do not delete your DK. Trust me. I kept forgetting how much I loathed nightblades because I kept deleting them. Then I would remake them and start over having forgotten how much I do not enjoy playing them. The starting over is the worst. So I now have one of everything and a baby tank and a baby healer, in case I ever have time to focus on either of those roles. And if I ever get bored with my DK's and want to work on something else for a while, I have a magic and stam version of everything. Lots of choices.

For 1. (enemies walking through walls) it depends on the area, there are some areas that for some reason it seems mobs can ignore walls and other blockage, but it isn't always like that, sometimes you can rely on walls or other to protect you and wait for them to come to you.

But, yes, once they're engaged with you, their ranged attacks have perfect tracking and aren't blocked by anything. Only thing that stops them, is getting outside of their aggro range so they'll reset or kill them.

For 2. (players running on mounts) It's an animation glitch that seems to be sometimes triggered when you quickly mount and start sprinting very quick afterwards (I think...) I triggered it once or twice myself without trying, but there seem to be a way to trigger it at will, and some people like it, it seems

For 3. (bots) it'd be better to report them, not that it'd make any difference in the short term, but if enough people report them at least we're letting ZOS know that we want them gone from the game. If we do nothing, nothing will change.

For 4. There are (generally) more than one way to play a class... if you don't like your DK (for some reason you didn't specified) maybe you can spice it up by changing abilities and/or the build you're using? You can try to go to the opposite spec, for example (if you're magicka, try a stamina build, or viceversa) and see if that's more to your liking? Also, you can try changing abilities and see if there's other bar combinations that makes up for what you feel are the classes' shortcomings?

Other than that, yes, the only way is to level another character... but, you'd only have to reach level 50 with it, once you're there, you'll inherit the champion level of your main and he'll be able to wear any max level gear you want to give him.

EDIT2: And also, Welcome!

EDIT: few minor typos (can't blame autocorrect, sadly )

Lol, I just logged in and my toon stuck halfway in the ground and moves like it’s on the mount but in the ground? Haha what madness is this?

So, I joined in to PvP to try and earn the assault skill line. First time in pvp on ESO. Dying isn’t a problem. Being 1 shot with an arrow or having 5-6 players gang up on me is. I’m in the Ebonheart Alliance and seems like not too many players play that alliance since I’ve only seen 1 other player the entire time and I’ve been on for hours just in pvp. I’m almost rank 4 for the assault skill line. Looks like for Vigor and Caltrops I need to be at least rank 6. It’s taking forever to level up. Some of the repeatables don’t give a whole lot AP. Traveling is a pain even with Rapid and it’s really disheartening to get to the location for let’s say “scout” just to have some player kill you in one a hit and you have to start your travel completely over. Overall the mechanics behind pvp are cool but seems like there are some major balance issues. Needless to say, once I’ve unlocked the two skills I’ll never EVER return to pvp again.

Welcome to the weird and wonderful world of ESO. May you spend many happy hours here.

Your questions have been answered in previous posts, so the only thing I'd add is don't delete a character unless you absolutely need that slot. DK is not a bad class to play, whether stam or mag and one day you might want to revisit it, so it would be better if it was still there. The most important thing, however, is to have fun.