Question about shield overkill.

For weapons that are completely blocked by shields: if the target's remaining shield are less than the shield damage from a weapon hit, will that hit also deal hull damage, or just kill the shield.

EX: If the target has 15 shield left, and it's hit by a Projectile Cannon at 70 Hull Damage/20 Shield Damage (upgraded), will that hit just wipe out the shield, or will it also do 70 damage? (also for bigger weapons, 90 shield/hit by Particle Turret, etc)?

Not quite sure. I do think that in your example it would also do hull damage since the shield damage is enough to lower the enemy shield to 0.

I think it depends on the type of weapon. For projectiles, I think the projectile is fully stopped by the shield, but beams seem to do at least some damage after penetration.

Projectile weapons will destroy the shield, and then continue their travel, but at a slower pace (depending on how much of their power they need to pierce the shield). The slowed projectiles will also do less damage - in your example, the bullet would do 1/4 of the damage, because it already used 3/4 of its power to pierce the shields, so it would do 0,25*70=17,5 (no idea about the decimals) hull damage.

This can be easily observed when using Mega Plasma Cannons.