What is Urgot's most reliable build when he mainly has to make nearby space truly unplayable?

The most reliable build on Urgot 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 Urgot's build instead of simply pushing the standard control bruiser 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 Urgot choose when the real issue is mainly staying in contact until the execute?

stasis is often the most stable choice because it directly supports this game plan: stay in fights long enough to apply dps and reach execution threshold.