My theory is that Imu gave the Holy Knights the regeneration power because of Vegapunk's broadcast and at Garling's request.
The WG is desperate. The Marine isn't enough as a defense. Shamrock wanted to recruit Loki as a Holy Knight. Sommers wanted the giants to work for the WG as soldiers.
The people and countries will rebel against the WG because of Vegapunk's message. There are still the Yonkos, who might form an alliance to stop the WG from flooding the world.