[RPG] What damage type does Force Sword do

While it doesn't seem to be explicitly spelled out anywhere, the Sneak Attack entry says:

Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.

This is extra damage dealt by the weapon attack; therefore, it should deal damage of the same type as the weapon. As backup for this, the section on critical hits says that:

For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.

It seems clear that Sneak Attack is extra damage added to the weapon attack, and therefore has the same damage type as the weapon.

Specifically, the Spy uses a shortsword, which deals piercing damage, so his Sneak Attack damage will be piercing also.

No, force damage does not affect creatures on the Ethereal Plane.

As quoted in the question, the DMG states that:

solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal. The exceptions are certain magical effects (including anything made of magical force) and living beings.

This specifically relates to movement. Force damage is not the same thing as a "thing made of magical force." This is confirmed in an unofficial tweet by Jeremy Crawford addressing this specific question:

Q: Do Magic Missile and Eldritch blast, which do force dmg., affect creatures on the ethereal plane, if they can be perceived?

A: No general rule causes force damage to pass from one plane of existence to another.

So even if you can perceive a creature on the ethereal plane, you cannot damage them with force damage.

Here is a later tweet response which states the case even more explicitly:

Q: Then what is the intent of DMG p. 48 "including anything made of magical force"? If no damage, would EB push cross planes?

A: Ethereal Plane: the DMG refers to things, like wall of force, made of force, not to the force damage type