Correct, alignment was not a factor for single-phosphor tubes.

The eye is not nearly as sensitive to red and blue so those colors did needed higher energies. But I doubt that it was because the phosphors are cheaper. Computer terminals and monitors were very expensive back then, but commercial color television made some of the common components (like phosphors) relatively cheap.

Green was found to be readable and pleasant. It was the most common for IBM equipment (such as the PC monochrome display).

Amber is very visible, especially in bright light. It was popular too.

White was popular with DEC terminals.

Right. They were monochrome hardware, so color alignment was not a factor.

If I recall correctly, those green phosphors were long persistence, so they could have a lower refresh rate. Amber and white phosphors came along later when it didn't matter anymore because logic got faster. Some of the first terminals I used had core memory, so they still retained their contents when turned off.