Thank you very much! & Questions

[Gildor]

Thank you very much for this incredible program!  I am really glad I found it.

You are welcome

1) Is it possible to change FOV?

No. Why model viewer should change FOV?

2) Is is possible to make the background plain white and plain black, or specify a color, instead of the current gradient?

Not now. I have plans to add some configuration file which will allow to setup some parameters including background color(s), visible axis, bloom etc. I think you want to produce a screenshot with transparent background?

3) Is it possible to move the light source?

No.

[Gildor]

4) Many textures are square. Is there a way to know what their proper aspect ratio in the game is supposed to be?

All textures are square
Game engine can map texture on the square, rectangle, triangle ... on any shape. But inside packages all textures are (usually) square.
Real restriction for texture dimensions is: texture sizes should be a power of 2 (64, 128, 256, 512, 1024, 2048 and so on). This restriction is gone with modern hardware, but usually texture sizes are powers of 2.