Is it possible to increase a resistance to a point where that element actually starts to heal you? For example, if you get very high poison resistance, will it eventually start healing you at a certain point..or does the resistance amount max out at a certain point. I read a comment in chat where "poison" was healing him, so I was curious about this.
No, due to diminishing returns on resistance values (each additional point is worth less) you can not reach 100% resistance to any element through that. I believe it's close to impossible to reach above 85% or so.
That said, there are certain legendary items that will make you immune to a certain element and instead cause that element to heal you. For example:
Countess Julia's Cameo - Arcane
Mara's Kaleidoscope - Poison