What is Singed's most reliable build when he mainly has to make the enemy fight lose time and structure?

The most reliable build on Singed 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 Singed's build instead of simply pushing standard AP?

Weaker vs high burst or ranged poke denying his access to fights.

Which enchant should Singed choose when the real issue is mainly keeping uptime in the useful zone?

gargoyle is often the most stable choice because it directly supports this game plan: maximize time in fights while applying constant poison and staying hard to kill.

Singed Wild Rift Best Build Guide (Patch 7.2a)

Singed Wild Rift Best Build Guide

Continuous disruption build: Singed is not worth a burst moment, but the sum of the paths he breaks, the chases he perverts, and the fights he forces to last too long. The build must therefore make his uptime credible before trying to inflate his damage ceiling artificially.