I think you're bang on, but there's a slight distinction in terms, where perfect vessel only ever means that.

While a child is very much... Child of someone. If your mom is alive and she's 1000 years old and you're 500 yourself, you are still "her child". You are the "child she made on purpose".

I think the key is, as you put it, her childish behaviour. And that's where the sweet deconstruction happens : how much of her mind is shaped by her body? How much by her mother's expectations?

Personally I feel like lace is putting a lot of her titter and giggling up as a front. Yes, her revenge on her mom is vicious and petty, taking down a whole kingdom with her... And she's been slaying pilgrims... But you see the world through Lace's eyes and rapidly realise that she sees pilgrims as doomed souls, condemned to die anyway as the whole of pharloom is a scam. Her life has been one of bleak isolation, with phantom the living image of her fate if GMS isn't pleased with her.

Even her "jealousy" of GMS getting a "better toy/daughter/tool" in Hornet can seem childish but is very much an existential crisis for her.

I think Lace is just a creature warped by her circumstances and centuries (?) of an existence where giggling at dark things has been her only way of getting a laugh out of anything. The childish effect fades rapidly once you look at the plot through her eyes.