Take another look at the original sentence:
You wanted to fix it this way:
To delete the comma between is and in would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two complete sentences joined with no punctuation whatsoever. Gustavo can tolerate any lecture ... begins the first sentence. In his mind, Gustavo imagines ... starts the second sentence. The spot between is and in needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.