AFAIK you can't use Lightsaber Block against an attack with Overwhelming Force, and Cortosis is only when using Lightsaber Block.

Not saying you're wrong, because I sure don't know, but that's not the way Overwhelming Force reads:

(Force; 1 Force point) When this character uses this Force power, its attacks (or damage from its attacks) cannot be prevented or redirected this turn. Enemies cannot use abilities such as Parry or Lightsaber Block to avoid damage, Damage Reduction has no effect, and abilities such as Draw Fire or Bodyguard cannot force this character to attack or deal damage to a different character. Likewise, abilities that reflect damage back to the attacker have no effect against this character’s attacks. Overwhelming Force does not affect damage from effects that are not attacks, such as Force Lightning. This Force power works even when this character is making an attack of opportunity.

It says you cannot use Lightsaber Block "to avoid damage", not that you can't use it at all. Similarly, it says that abilities to reflect damage back to the attacker "have no effect against this character's attacks". In both cases, it essentially says the abilities have no effect, not that they can't be used. Of course, there's usually no reason to use an ability whose effect will be ignored anyway, but Cortosis Gauntlet gives a reason. They'd still take the damage from the attack even with a successful block, but could invoke Cortosis Gauntlet in the process. Does the FAQ mention it?

Yep, Sithborg is correct (not that there is any doubt) as I asked this on the main rules forum at WotC a long time back and NN responded in kind.