The spell saying that the shadow blade dissipates shortly after you drop it or throw it creates the specific logical consistency that the caster must be in physical contact with the created tool to maintain it's existence. Being able to give the shadow blade to someone else is a cool idea but it does ignore the deliberately established logic of the spell description.

Can magic weapon be applied to Shadow  Blade? This does take two people.

Can you use a higher level spell slot to make it more powerful

When you cast this spell using a 3rd- or 4th-level spell slot, the damage increases to 3d8. When you cast it using a 5th- or 6th-level spell slot, the damage increases to 4d8. When you cast it using a spell slot of 7th level or higher, the damage increases to 5d8.

Rogues do gain proficiency though as it's a simple weapon which rogues are proficient in.

Do you mean rogues who did not cast the spell? Because we can see the caster is proficient regardless of their class.