You have a subpanel, no there is no bond there (which is correct, bond is back at the main), so the neutral and grounds should be kept separate.
Yes, the ground wire should go to the ground bus. Looks like you might have enough slack to simply move it, you might need to shuffle a few ground wires to free up the closest position.
Check the other end of the circuit, the wiring at the receptacle. The ground wire should be landed on the green terminal, and the black and white should be on the brass L1 and L2 terminals. If the ground wire was landed on a silver terminal and is being pressed into service as a neutral conductor, then don’t let your builder ever do anything electrical for you again. If that’s the case then your receptacle (and welder?) need a neutral, and you need one more conductor. Not fun.
Yes, you should correct the trimmed neutral. Wire strands should never be trimmed back to make things “fit.” Your panelboard manufacturer probably offers a lug kit that has a larger ~1/0 barrel lug with prongs that fit into two consecutive bus bar holes, that’s the right way to fix this. Splitting the wire into two bundles is also a reasonable field solution, better than what you’ve got now.
The black goo is antioxidant gel. It should be used on any aluminum termination.