What is Shyvana's most reliable build when dragon form truly has to swing space?

The most reliable build on Shyvana is the one that preserves the first spike while still keeping an escape valve. If your item order prevents you from playing the champion's first real window, the build may look strong on paper but it is not reliable.

When should you truly adapt Shyvana's build instead of simply pushing her standard ramp into dragon form?

You should adapt as soon as the enemy draft stops you from playing the timing the build was meant to serve. If you need to survive longer, absorb more poke, or answer a specific engage pattern, the purchase order must move before you even think about the final six-slot build.

Which enchant should Shyvana choose when the real issue is mainly holding after dragon-form entry?

stasis is often the most stable choice because it directly supports this game plan: stack hybrid stats to stay in melee and convert dragon form into fight advantage.