Keeps 3 days refrigerated. The vegetables soften on reheating — use a hot pan rather than the microwave to keep some bite. Don't freeze; the vegetables turn watery.
Too much in the pan at once, or wet vegetables. Cook in batches, and dry the vegetables properly after washing.
No. A large heavy skillet is fine — what matters is high heat and not overcrowding. A wok just makes tossing easier.
Slice everything up to a day ahead and refrigerate in separate containers. The cooking is under 10 minutes, so the prep is the whole job.