1. Looks like that's the muffler and looks like they've drilled a small hole so that any condensate that starts to flow down the muffler to the exhaust tubing gets blown out rather than pooling up down in the tubing at the lowest point and rusting it out faster. Mufflers are the coldest parts of the exhaust and the last thing to heat up properly to burn off any residual condensation from burning the fuel so if you take long rides and heat up the muffler it should burn off condensate, if you take short rides or don't heat up the muffler then that weep hole seems to be there to help blow out condensate traveling down the exhaust tubing from the muffler.
2-5. with regards to metal shavings, china bikes have the most amount of factory garbage left in the motors after assembly, usually metal flashing, shavings, etc. Just keep changing out the oil after getting the metal into suspension and draining it out and changing in fresh. Royal Enfield brand has the cleanest engines with the least amount of contaminants found in the oil in the motors from the factory, followed by the Japanese brands, then middle to lower end is the European brands and dead last and off the scale are the China brands. I don't own your bike or motor but there will likely be a screen inside the engine that you can trap larger metal particles and you can take that off and empty it of any metal/flush it out and then reinstall it with each oil change.
7. Oil level indicators are read with the bike upright and not leaning over to one side or the other. If you have your oil level in the middle of the sight glass between H and L you are good. You will notice if the bike leans to one side during a turn it will be closer to H and when leaned over the other side in a turn it will be closer to L but it will always be within the acceptable range.
8. First and foremost, always in the background be contacting the place your bought the motorcycle from and also keep them informed of what you are experiencing and making sure they know the same links to the videos, etc. I don't know how long your warranty period is, it could be short like 90 days or so about so don't tally from telling the seller right away. Now it is impossible to tell from just a video but if I was to hazard a guess I would say the clatter to me sounded like the clutch plates snapping into place or something else in the area. The only way to tell where a noise is originating from is to use a stethoscope. Basically mechanics take any large long flat bladed screwdriver and shove the plastic handle against one ear and push the metal tip onto various places on an engine, or transmission and listen for the noise and wherever it sounds the loudest through the stethoscope that is the area where the noise is originating from. You are also starting the bike with the handlebar fully cocked to the left. Unknown is if you have the throttle cable set up correctly so that when you turn the handlebar full left and full right it doesn't change the throttle cable pull on the carburetor. Otherwise turning the handlebar and accelerate you when you don't want to and can be dangerous. The amount of free play in the throttle should stay the same from full left to full right and inbetween, it should not bind or pull harder on the cable at one position or the other. Same with the clutch cable, you don't want it binding or pulling when the handlebars pull full left or right, you want that cable to have the same amount of freeplay through all positions of the handlebar.
10. The engine oil is sloshing around inside the motor, having bubbles after running the engine is normally not an issue. Heat cycles turn oil dark and some additives in some oils will also darken rapidly from oxidization and heat accelerates the oxidization process. What turns oil eventually black is carbon deposits getting into suspension from the engine working and burning the fuel.
I didn't read anything about using the choke or setting idle. I have no idea what carb your bike comes with so I am guessing some sort of PZ clone.
PZ clone photos:
Full choke, use this with a touch of throttle to keep the motor running. Carburetor motors run very slowly when cold and as they warm up the idle speed keeps speeding up, that's just the way they are. You keep the idle set correctly at cold with your hand on the throttle until the engine is running hot enough to have sped up to the correct idle speed for whatever the idle speed screw setting is on the carburetor is set to.
Half choke, you switch to using this after full choke while still keeping the motor running with your hand on the throttle.
No choke, you switch to using this after half choke while still keeping the motor running with your hand on the throttle. After your motor is up to running temperature you can roll off the throttle with your hand and the motor should be running on its own. If it is not and it stops then you need to adjust the idle speed screw.
Idle speed screw, there are no set settings, you turn the screw in clockwise normally to increase the speed of the idle and you screw it out counter clockwise to decrease the speed of the idle. You set this after the motor is up to running temperature.
Air/Fuel mixture adjustment screw. On OEM carbs this usually is hidden behind a plug to make the California Air Resource Board or some other government killjoy happy. After you remove the plug you have access to this screw. It is used in conjunction with the idle speed screw to set the optimum fuel and air mixture rate for idle speeds. Usually these are approximately 2 1/2 turns out (counter clockwise) from turned all the way in (don't jam it hard, its brass and could strip). If you buy after market carburetors these are usually not plugged and easier to access as you don't have to go through the process of removing plugs.
