There was a collective effort to understand how the gold works a few threads ago (refer to the middle section of pic related) and I'd say that there are reasons why it wouldn't work.
First, the alloy is in fact inclusioned, it's the whole reason why it can morph. Second, the peculiar way in which it originally bonded to Phos makes it so that the alloy runs through all of his body: when it stretches, it damages a bit all of the mineral it runs through, not just the phosphophyllite -that one just has it worse because it's more brittle.
Still, what causes the damage is less the gold itself (which is 2.5 Mohs, even less hard than phosphophyllite), but the force that has to tie Phos's body together when the alloy streches outside his body. When he makes a full external construct or when he stretches forward several meters high while being anchored, the semi-solid alloy is subject to tremendous static force that has nowhere to go. Same when he stretches it very thin.
Why wouldn't your suggestion work then? Because if he made a gold "armor" for his torso it wouldn't really change anything when it stretches but put the torso under *more* strain. (plus it'd be shit at protecting him from blunt hits). Even worse, smashing a gem's torso effectively incapacitates them -Phos wouldn't be able to work like that. He's been shown to be *sort of* able to use the alloy like a nervous system, but he always needs to be in one piece to do that. When he's broken at the torso, like in the fight where Ghost was taken, he couldn't use to alloy to reconnect his two halves. The only exception where the alloy kept working in connecting separated pieces was the psychotic breakdown and even then smashing Phos in the torse ended it.
So no, that really wouldn't work.