Using Inferno is part of the attack declaration, not a damage effect.
Before even rolling the attack roll, you'd declare you were using Inferno, and that you are replacing SRM X with HT of X.
The HT would then apply, on a successful attack, to the unit as damage effects usually do, at the End Phase.
Also, specifically for heat, all heat adjustment is done in the End Phase, regardless of how it happened.
Heat Seeker would not get a bonus, because the HT effect hasn't taken effect.