Are plastic enclosures safe for hobby mains voltage projects?
For a plastic enclosure, you want to treat any metal stuff coming out the same way the DC output of a doubly insulated supply (e.g. wall wart) is, meaning it has to come from the side of board that's separate (to standards) from the mains. Separate to standards means clearance, creapage etc. You can look at some teardowns to see what's involved; here's one to medical standards: https://www.youtube.com/watch?v=_EMAggN5H-w

Honestly, I'm not sure why you want


  a switch and some sort of optical indicator perhaps with metal bezels /lever


(emphasis mine) There are plastic alternatives for these, and even with insulation rated for mains [and above].

As for


  there will be a mains transformer, with a bolted attachment through the case


This might be tougher to find alternatives for in DYI projects, but I've seen 50VA (mains) transformers held in place by nothing but plastic tabs, but this in specially/factory designed plastic cases where those tabs have the necessary thickness etc. and are part of the case body. So, how heavy/big is your transformer? If you can sacrifice screw thickness (or overbore the holes) without sacrificing mechanical endurance, you could use a solution similar to mounting TO-220 isolated on heatskins, i.e. insulating "shoulder" bushings also know as
"nippels" in some parts of the world. Full-bodied nylon screws also exist, but you'll probably want to read the manufacturer's data carefully to make sure they are strong enough.