Final answer:
The provided excerpts do not contain information about Jeannette Walls or her book 'The Glass Castle', thus it's impossible to detail the incident that cements her conviction to leave Welch.
Step-by-step explanation:
The incident that helps to cement Jeannette's conviction to leave Welch and her family in the book The Glass Castle is not directly mentioned in the given excerpts.
These excerpts do not pertain to Jeannette Walls or her memoir. To accurately answer the question regarding her conviction to leave, one would need specific context or a passage detailing an event from The Glass Castle. Typically, such a conviction could be influenced by a defining moment that clarifies the need for change, such as an extreme family situation, a significant disappointment, or a realization of the potential for a better life elsewhere.
Without the appropriate reference, providing an accurate and detailed response to this question is not possible.