A101 - According to the rules, this is acceptable. Bull rush is fine as part of a charge, and they did provoke the AoO.
Q102 - If I cast Mantle of the Icy Soul [Spell Compendium] on myself to get the Cold Subtype for Immunity to Cold, and I already have the 'natural/granted' Fire Subtype for Immunity to Fire from using Mantle of the Fiery Spirit [Sandstorm]. Does one cancel the other out, or do they stack?