Why Japan could not rapidly expand its naval pilot pool

Did Japan fail to reach the critical number of trained personnel needed for rapid pilot expansion?

Yes, but the deeper problem was institutional: Japan had not built the training infrastructure or systematically returned enough veterans to instruction duties.

The bottleneck was a force-generation problem rather than a simple head-count shortage. A larger reserve air arm needed schools, instructors, aircraft, and a deliberate system for using combat veterans to raise the standard of new pilots. Without that structure, building convertible carriers could not produce usable carrier air groups by itself. Ship conversions and aviation personnel policy therefore have to be judged together.