What is Kassadin's most reliable build when he mainly has to connect useful patience with real space denial?

The most reliable build on Kassadin 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 Kassadin's build instead of simply pushing his standard scaling route?

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 Kassadin choose when the real problem is mainly entering without losing the whole exit?

stasis is often the most stable choice because it directly supports this game plan: survive early then dominate late game.

Kassadin Wild Rift Best Build Guide (Patch 7.2a)

Kassadin Wild Rift Best Build Guide

Late-swing scaling assassin build: Kassadin does not win because he passively survives to late game, but because he reaches a breakpoint where the map suddenly becomes too small for enemy carries. The build must therefore connect his patience phase to that mobility explosion without buying him an early game he will not truly play.