I installed KMEM (and KLAX) trials today. The first run went fine. I restarted P3D because the 10 minutes went by too quick before I could explore it.
It's 5-6 minutes. And it's per-session. You can restart the sim, and have another Trial, and so on. It never expires, it's just you cannot fly for more than 5-6 minutes in the SAME sim session.
The second run was worse: everything was flat or in the ground and no buildings showed up. At LAX, only the jetways showed up, but nothing but some textures on the ground at MEM.
You probably restarted the sim too quickly, not allowing enough time to close all the opened programs and files. A Windows restart would probably fix this.
Another question as well: are there non-cargo gates that can fit the B777? All gates I tested were too small for the plane
No 777 passenger ever goes to KMEM, only the cargo ones. Most of the planes used for passenger traffic are fairly small ones.