Should Tristana really rush IE when she mainly wants her jump respected as a zone threat?

The most reliable build on Tristana 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 Tristana's build instead of simply pushing standard explosiveness?

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 Tristana choose when the real issue is mainly jumping without losing the rest of the sequence?

quicksilver is often the most stable choice because it directly supports this game plan: create early advantage and convert into structural pressure and chained kills.