If it is level 39, can it still evolve?

No, the laser level is generally going to be much more accurate and helpful. A spirit level is fine, but it's not what professionals use anymore.

Yes, unless it's holding an everstone. That's the only way to stop an evolution besides pressing B during the evolution.

It evolves at any level other than level 100, as long as you're in Mount Lanakila.

Yes, it will continue to attempt evolution until it reaches level 100.

It is impossible to evolve Cubone into a Kanto one in Sun and Moon, but if you evolve Cubone into Marowak from another game like X and Y and transfer...

Yes. Like other Pokemon, it will attempt to evolve every level until it reaches level 100. Of course, your 3DS has to be upside down for it to work.

Magikarp will evolve into Gyrados once it gets to level 20. This is the same for almost every main series game.

I would avoid taking extra supplements if your levels are in the normal range.

Beeswax would affect the hold of the pomade, and oil affects the shine.

Priests can only equip Maces and Daggers as weapons. Dagger is the preferred weapon, as it is usually famed for its high dps.