I received the other piece for the motherboard tray, and fits much better. The motherboard now can be mounted straight. Here's a view of the motherboard side. It's best to remove the top before installing.
When I installed it the first time, the PCIe riser cable fit in very well. The length is just right for the case, with just enough extra length for moving it in place when installing. But it was much tougher to remove it. Either one end could be pulled up, or the other, but I had to pry it a bit with a long tool to get both sides to come out easily. Best way I think is to move the motherboard slightly towards the bottom before screwing it in, then it'll be easier to wrap the cable around.
Here is what the top view looks like.
The components fit straight, but you may notice the tray piece flexes a bit. That's to be expected with this material, many areas (especially the top case side) are flexible. But it all feels very sturdy when the parts are installed.
I also believe the standoffs offset the motherboard a bit too far towards the side of the case by 1mm. May cause imperfect alignment with the I/O shield (which I don't have for this board). A motherboard tray where the screw holes are recessed by 1mm would solve this, and won't make a big difference in the sturdiness of the standoffs.