Does the fact that Regina is still ipinapakita signs of redemption without her puso ipakita that she truely is changing or is it poor writing?

Regina's different. Even without her puso in 3x03, she told Tink how much she genuinely loved Henry with a undeniable, loving look on her face.

Snow also sinabi in this episode that she feels “deep” feelings with or without her heart. That Gina also feels things within her soul. I think that's beautiful! ;)

And the fact that Regina's puso is “resilient,” according to Belle, that it can someday be pure again! <3

I think the difference in behaviours between heartless Regina and Cora is due to a few reasons.

First is how long they'd been without a puso - Regina was missing her puso for a few weeks, maybe a month, while Cora had been heartless for decades (longer than Regina's entire life!). In general, characters who've Nawawala their hearts for years, like Cora and Graham, display this emotional numbing effect, but characters who only have their hearts out for a shorter while have less of an effect.

The reason they took out their hearts might also play a role. Cora deliberately removed her puso to numb her own emotions. Regina took hers out to physically protect it. Maybe because Cora wanted the numbness, this amplified it.

Another possible reason could be the individual characters themselves. I always thought Cora was a bit metaphorically heartless anyway. She abandoned Zelena and left her to die long before she'd removed her heart. Maybe characters who are madami emotional have less of this numbing effect.