i have had the good fortune to acquire a "rising sun" amulet, and at level 76 im absorbing 57% fire damage, this had me wondering; if i lowered my fire resistance as much as possible whilst wearing this would i benefit from more healing or am i missing something obvious that would make this a stupid idea
No, you don't. Resists are applied first, so the less life is left to be absorbed, the less will be healed on top of the absorption. Resists are good if you would otherwise suffer more damage than the amount of absorption.
In previous seasons (I'm not that far in this season yet), my sorcs could do hell Meph runs with negative fire resists and a Rising Sun amulet and be healed from Hydras of Councils and meteors and firewalls of Bloodlords. Fireballs might deal a bit more damage, but the amulet buys you enough time to kill the Bloodlords, even if there's a conviction boss.