1) Note it doesn't matter much if your design distorts by 2-3% (very much as for thermal distortion). Take a LQFP100 package. The line of pins is are about 12mm long. With 2%, that's 0.3mm, still within in width. That's 0.36mm difference, with central pin centered the edge pins will be 0.18mm off. That's still pretty acceptable. And yes, with 20cm PCB that will be 6mm difference, but you don't use 20cm long parts - your whole board dimensions may change but parts will still fit and tracks will remain where they should be. And if you use the same kind of transfer medium for both sides, both will be off, but both by the same amount so they will match each other. At worst if you make an edge of the board into a PCI-like slot connector, it may be slightly off... but you can always fix that by pre-scaling: Print a 10cm bar, iron it onto the board, measure it and if it's 2% bigger, scale your PCB project image making it 2% smaller before printing...

2) The transparencies used for transfer are either for laser printers - resistant to temperature, or for inkjet printers - and these get soft with temperature, so ironing them hard against the surface will make them stick and leave the toner, if heat makes them flow, the toner will remain against the copper anyway. Paper heat distortion is near zero, copper of the board is firmly attached to glass fiber which doesn't change dimensions either (so heat causes material stress but the distortion is really small).


To author of original post: If you have an inkjet printer which can print on rigid medium like the PCB, you might consider cartridges filled with etchant. Insert the board, print, wash, done.