Obligatory not an expert, but from your picture
It doesn’t look like zircon, nor like French cut on the side stones. Them not being French cut is no big deal - but I’d think we see at least some tell tale zircon doubling in the facets?
And, again, from what I see in the picture the side stones look like they are barely held in. I see a prong on one of the four corners. I’d be more worried about that then your concern about breaking the zircon.
I’d guess more towards retro jewelry period, but late Art Deco as well.
Anyone can take out an original stone to a plat mounting and put subs in. Nefarious or not.
Diamonds I would consider old mine cushions.
Can hazard a guess to the center as being any type of simulant /synthetic, to topaz or aqua?
Is your jeweler only resizing or are they rehabbing the prongs as well?