What is Dr. Mundo's most reliable build when he mainly has to gain ground by being too annoying to remove?

The most reliable build on Dr Mundo 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 Dr. Mundo's build instead of simply pushing raw health and regeneration?

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 Dr. Mundo choose when the real issue is mainly staying expensive under prolonged focus?

gargoyle is often the most stable choice because it directly supports this game plan: absorb damage and survive indefinitely.

Dr. Mundo Wild Rift Best Build Guide (Patch 7.2a)

Dr. Mundo Wild Rift Best Build Guide

Frontliner build for attrition and refusal: Dr. Mundo wins when the enemy spends too much time and too many resources to finally make him manageable. The build must therefore protect his pre-fight presence, late entry, and the real value of his regeneration.