I wouldn't worry about it. If you damage the tubes taking them apart it should be obvious at the time. Heat damage will likely come in one of two forms:
1) destroys the heat treat of the tubing, this is unlikely because the vast majority of bikes are not HT. Heat treating can do positive things, but the likely scenario would include, distort frame, make it impossible to bend straight, and overheat the brazed joints. For this reason, like gun actions and barrels, bikes tend not to be heat treated.
2) Burn up the metal. This can happen and is more likely the thinner the metal. Other than heating the parts up carefully, you can also:
a) Use a reducing flame, this will actually de-oxidize tubing, to a point.
b) Put a coating of flux on the parts to be heated, this will protect them from oxidation when you move the flame around.
c) You can break off some matchsticks into the drain holes above the drop,or vents inside the crown, tip that wood up to the crown and plug those holes with something that won't burn out. This will smoke the inside of the fork blades which is similar to back purging the tubes.
Other than the reducing flame, most of that stuff is probably not necessary, if the parts look normal, and require only normal clean-up when you are done, you should be good to go.
For removing the steering tube, if the inside of the tube is accessible, from the underside, or if you can cut some off and still grip it, you can heat the inside of the tube so the heat only rises to the point that the braze yields in the crown. Thereby not affecting anything you are keeping.
A lot of this comes down to the overheating boogeyman the bike industry spread years ago to discourage cheap manufacture, and business shift. They won that BS argument, but no welder, blacksmith, gunsmith, toolmaker, or knifemaker would take that nonsense seriously. Don't take heat treating advice from an industry that basically doesn't heat treat. Remember, those bikes TIGed together at 3000 degrees F don't actually exist.