non-flamable insulation would be tricky as most are foam based. people definitely insulate their boilers but usually only when they have one with an electric element inside like a converted bucket or some such. I would be wary of any insulation being near a hot stove top. I sanitise with the dishwasher too. I'm not sure if the temp of the actual wash matters, because the drying phase is usually enough to sanitise, and at least on mine that's the same temp regardless. I may be wrong and you're safer with the hotter temp, but my fastest cycle is also one of the cooler ones. I have never had any problems with that.
If your pot is wide enough you can sit it over two rings. Also you can try using some 'boil concentrators' , that is, chuck 2 or 3 copper coins in the boil.
The cheapest option for a better boiler is to get one of the fermenting buckets from the homebrewco, and add the element from a cheap 7.50 argos kettle. would cost you not much more than 20 euro, and should boil 20 litres well enough. you could even add 2, or go for the larger fermenter (33l), or even get a 60l boiler from hop and grape and add a burco element, or , or, or...
deciding the right system gets seriously obsessive!