Will 3.5" polyiso keep my electrical cables from melting?
A low humidity sauna might be as high as 100°C measured above the upper bench.  It's really no consolation that much of the wire will be behind insulation and won't get to the full temperature inside the sauna;  if the insulation is compromised in the boxes, that's hazardous.  The connections in the boxes may be rated for lower temperatures than the 90°C insulation.  

Keep in mind that the wiring generates some heat of it's own.  The ceiling light in particular would concern me.  Heat rises, so it will be hotter than the average temperature in the room.  The light will generate some heat too, on top of the ambient temperature of the sauna.  

I don't think receptacles in the sauna is a good idea, receptacles are generally rated for 60°C operating ambient temperature.  You might find some industrial specialty receptacle made to withstand high temperatures but I bet it would cost a fortune.  

I think you could safely blank off the and insulate over them, leaving an insulated access panel cover to get to them.  (You can't bury a junction box, it has to be accessible.)