Doctor Fox opened his eyes and let out a peculiar cry. ‘Ah! Then you will
be out of the fog. At forty you can no longer harm anyone, and no-one can
‘What?’ she thought. ‘That can’t possibly be true.’ But it was exactly the
kind of answer she had wanted.
He laughed and pulled a long face at her, then turned to kiss the child
goodbye, but the boy’s face was suffused with sudden bliss, and he flung
open his arms as if the vision splendid had shimmered into view over his
father’s shoulder. Elizabeth too recoiled, as we do in respect and fear before
the ecstasy of someone tripping on acid, to whom we are nothing more
substantial than a liquid blur of light.
First of all I want to know if I am right in understanding the phrases in bold.
Dose "But it was exactly the kind of answer she had wanted." mean: But it was exactly the kind of answer she had liked to hear when she was younger?
in the sentence "as if the vision splendid had shimmered into view over his father’s shoulder" instead of "vision splendid" can we write "splendid vision"? I think "splendid" is adj and "vision" is noun. And does "vision splendid" mean: something splendid that we see?
It's hard to be sure what "you will be out of the fog" means here without more context. I would guess that, as you suggest, it means "you will be out of danger". (The other thing it might mean, in some contexts, is "you will be able to see clearly", but the rest of what Doctor Fox says doesn't seem to fit that.)
I don't think "exactly the kind of answer she had wanted" is looking as far back as you suggest; it's the kind of answer she had wanted when she said she was nearly forty. (I think. Again, knowing a bit more about what happened immediately before might make change my opinion.)
It certainly means that he deliberately put on a sad expression. It might be to mock her, as you suggest, or he might have expected her to be amused by it too. Again, this might be clearer with more context.
I think "vision splendid" is that way because the author is referring to something else. It does indeed mean the same as "splendid vision", meaning something splendid that we see. Since Helen Garner is Australian, I think the most likely thing she's referencing is a poem by Banjo Paterson which contains the line "And he sees the vision splendid of the sunlit plains extended", referring to someone who lives out in the Australian outback.
(This sort of inversion is fairly common in poetry, probably originally just because being able to switch the word order around is helpful in getting your lines to have the rhythm you want.)
Respect isn't the same thing as liking. It's what you might feel towards a parent, a teacher, a judge, an expert in whatever field you work in, etc. I think the idea here is that she feels the boy is having an experience similar to someone seeing something glorious, or affected by powerful drugs ("tripping on acid"), and is aware that right now she is much less important to the boy than whatever it is he's experiencing.
The "blur" part certainly means unclear, but I think what she's trying to describe is more than simply unclear light: the idea is that in some way the light is (or seems to be) moving around like a blob of liquid might. Again, she's trying to describe the experience of someone under the influence of powerful hallucinogenic drugs; people in that state often find that their vision is distorted in strange ways.