This is something we have discussed, but ultimately would prove much too complicated to track and convey correctly. From April onward you’ll need to be in the same corp or alliance to provide remote assistance in a war.
The war would always last as long as the reinforcement cycle of a high sec Upwell Structure so significantly longer than 2 days, but you are mostly correct, this change allows corporations and alliances an avenue for ending a war as a defender, and generally speaking, larger groups are going to find this an easier challenge than smaller ones.
Mercenaries would be an option in this case. generally the high sec restriction is there to prevent destroying the HQ from becoming a nigh on insurmountable challenge for a high sec group.