Should Tryndamere really teamfight or mainly force the map through side lane depending on the build?

The most reliable build on Tryndamere 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 Tryndamere's build instead of simply pushing the standard crit side-lane 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 Tryndamere choose when the real issue is mainly entering without wasting his ultimate?

quicksilver is often the most stable choice because it directly supports this game plan: maximize side lane pressure.

Tryndamere Wild Rift Best Build Guide (Patch 7.2a)

Tryndamere Wild Rift Best Build Guide

Permanent-threat side-lane duelist build: Tryndamere wins when the enemy has to answer his side pressure while still fearing his entry into messy fights. The build must therefore protect his duel, his access, and enough map tempo for his ultimate to truly transform the exchange.