After immediately jumping at her with "cómo que no se usa así, está en mi libro de slang mexicano y es muy común y qué diablos me estás diciendo," I stopped arguing, listened, thought about it and realized she was totally right. WHY YES, I do totally overuse "es que" and, yes, I guess it really does carry the connotation of making an excuse for something and, yes, I totally am just using it as a transition (así que) when it should be reserved for excuse-making.
Also, I really do need to stop relying on "un montón." Mucho works just fine. At this level, it's all about diversifying your vocabulary. it's easy to stick with words and phrases that are comfortable and sound colloquial, but it's a trap. Hay que buscar, buscar, buscar.