Omega gold Milanese bracelet - fused or removable?

Yes it is fused and no it is not adjustable, well there are 2 or 3 settings in the clasp about 5mm apart but that is your lot. If it doesn't fit you it's a problem. Making it smaller is a job for a professional goldsmith. Making it bigger is possible but expensive and a job for STS who have the tooling to add the right mesh in. The plain back was indeed probably special order for inscription purposes, it is seen quite a lot on domestic production pieces.

With these welded on Milanese bracelets, in general, if it doesn't fit, don't buy it.

Is the watch already your´s? If yes would you like to wear it on a strap?
The bracelet could be removed and the watch head could be used on a strap because the case has lugs. Many watches with integrated bracelets don´t have lugs so they could be only used as paperweights without the bracelet 😉