The switch sounds like it should work great -
simple and robust! Note that it will be VERY loud unless you enclose it to muffle the BANG.
On the blast shield, G10 material is tough and should work - as long as you also protect the interior surfaces from erosion damage. Copper fragments progressively erode any interior surfaces that are on a direct path from the exploding coil. Fragments are mostly ejected radially around the work coil, and they can easily reach multi-Mach velocities. I've had to install steel plates to prevent shield damage and failure. I've successfully used 1/4" to 1/2" thick plates made of low-carbon steel and stainless steel with good success on the two sides and top that are in the direct path of the exploding fragments.
On the floor, I use a loose piece of one-inch thick block of AR400 (abrasion-resistant steel) placed a short distance right below the work coil. It has an upward-facing semicircular curved surface that faces the work coil. Its large mass absorbs the shock and momentum of the downward-traveling fragments, and it also protects the floor of the chamber. An unprotected floor of G10 (or other polymer or composite) will be rapidly chewed away by coil fragments. However, my work coil is only about 3" above the floor, so my floor likely takes more of a beating than yours will with 5-inch standoff insulators. I'd still recommend using a steel plate to protect your G10 floor.
I don't know if the ceramic standoff insulators will survive for very long inside the blast shield. I can't over-emphasize the destructive nature of the ejected coil fragments. Even though copper is soft, their high kinetic energy creates dents, and even small craters, on the surface of steel plates. The shock wave from the exploding coil and plasma is also quite brisant - not a good thing for nearby brittle materials. I'd recommend using a less-brittle material, such as UHMW or polyethylene, instead. These materials will flex or dent, instead of possibly shattering, when subjected to mechanical shock through coil mounting bolts, exploding-coil shock wave, and the torrent of high-velocity coil fragments.
Hope this helps and best wishes,