1. Blue flames are at a higher temperature. But in terms of variety, Shoto has the upper hand because of he has two quirks. And in terms of skill, Endeavor has more experience so it will be difficult to decide who's stronger.
2 & 3. I don't think they are immune to heat in general. Shoto did get a scar from hot water (heat). And, to an extent, they are also able to hurt themselves by their own flames. When Endeavor fought with that Nomu, he was slowly scorching himself because he was using his powers too much. I believe this applies to Dabi/Touya as well; as we see his skin looks like patches which can only mean fire wrecked his body.
Thanks for the response.
I figured Shouta got burned by the hot water because either 1. he was young and his quirk(s) hadn't fully matured yet and had it been mature it could have protected him from the heat. or 2. The boiling water to his face was a surprise so he didn't have a chance to activate his quirk to keep from being burned.
But you make good points.
Though unlike Endeavor and Touya, Shouta can control how hot or cold he gets by his 2 sides.
1. Does Shouta have blue flames yet? If not wouldn't Dabi be stronger?
2. Could Dabi's fire actually hurt Shouta considering Shouta's own flames don't burn him?
3. I'm not clear on this, can Shota or Endeavor touch flames not produced by themselves and not get burned.