What kind of temperature is the right heat when using a hair dryer?
I would just heat it until it starts to get a little soft, to the extant that you can just start to mold it to shape.
But if you call or email Risport, and list the boot model, they may be willing to list the heat molding temperature.
The problem is that that isn't quite the way they usually give directions. Instead they tell you the pre-heated oven temperature, and how long to leave it in the oven. That particular combination gets the gel, or whatever, up to the desired combination, but it is probably a higher temperature than your boot can safely take for the longer exposure it may take a hair drier to get up to that temperature.
BTW, I wouldn't trust a kitchen oven. Kitchen ovens can be off on calibrated temperatures by a lot more than the boot ovens, and may cycle by too many degrees.
I always try to be careful and cautious on such things. If you go too high, the boot may fall apart. My best guess is that a hair drier can't get that hot, but I might be wrong. (A heat gun can definitely get hot enough to do that, or to burn the leather.)
If your laces are not cotton, you might consider removing the laces, because they can melt too - I'm not sure at what temperature. OTOH, by the time you re-lace, the boot may be too cool to do you any good.
I flexed my ankles continuously while I was heat molding, in an attempt to break them in faster. It worked, but a while later, they broke down, so that may not have been a great idea.
I really think that talking to Risport is a good idea first - and letting them rebuild it, if they do that, would risk doing less damage. $60 or so plus shipping is a pretty typical rebuilding charge. They would remove the top stitches, pull out whatever is stiffening the boots, and put in something less stiff. It's pretty routine.