see, i would agree if mantra was used a lot in the story, but that's not the case. the only character who have been really in touch with it is coby, and that's pretty much it. that's how uninmportant that power is. so yes, allow me to believe that oda added the mantra part into haki because he wanted haki to be a credible power, more than just a power that allows characters to touch logia users. and yes, i think that oda didn't really know what to do with this power until shabondy/amazon lily. it seems obvious to me that logia powers are what decided oda to make haki a fighting power. finding a way to beat logia like crocodile or enel would get old at some point. and he probably didn't like the idea of using seastone during fights as well, since it is very underused and almost ignored. and that's probably how he got the idea of making haki a full power.
so now when you read that convo between ace and blackbeard, it's as if armament haki never existed. it is really in contradiction with pekoms saying that logia users don't feel invincible anymore once they reach the new world because everyone and their mother can beat logia users with haki.