Hi Don, your question has popped up in the Australian Houzz site. American cabinetry has a greater widespread use of wood and painted wood than Australia. I'm envious. I'm not sure you'll get very much useful info here.

You can paint laminate. Products at Bunnings and plenty of instructions on Pintrest.

Consider a burnished oil finish. I believe this to be superior to layers of polyurethane which often cause boards to stick together and show the scratches. It allows boards to expand and contract. If scratched just use oil over thecsctratch and it will disappear. This would still be done by a professional.

i don't understand all the concern..(obviously won't be cheap or easy and probably needs an engineer to design the frame) but the firebox probably weighs no more than 200kg (less than a bathtub full of water and a bather) and the flue is suspended from the ceiling or a power flue can be directed through the wall or floor ..the challenge will be finding the most suitable brand and model (preferably zero clearance) and keen tradesman to organise the steel frame as the timber shelves are only decorative not structural ...we have a 250kg corten steel courtyard gate that is only suspended from steel hinges and they not only hold the weight above the ground the door also has to move...consult the manufacturers who should be able to give more specific advice