What is Rell's most reliable build when she mainly has to engage without being traded back too quickly?

The most reliable build on Rell 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 Rell's build instead of simply pushing the standard engage tank 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 Rell choose when the real issue is mainly holding the zone after entry?

gargoyle is often the most stable choice because it directly supports this game plan: engage, absorb focus and lock enemies in place.

Rell Wild Rift Best Build Guide (Patch 7.2a)

Rell Wild Rift Best Build Guide

Collective-lockdown engage tank build: Rell wins when her entry turns space that was still playable for the enemy into a fight that is suddenly too tight. The build must therefore give her the path, impact, and survival required for her engage to truly open something instead of merely looking impressive.