1: Down-mixing, include LFE channel? 2: What's Fullrange speakers: Left and Right sound setting for?

1: When should I include the LFE channel when downmixing?

2; In Windows' Sound > Configure (wizard) settings, What is the option "Full-range speakers: left and right speakers" for?

Fin:

How can I tell when a set of stereo speakers benefit with any of these options?

The LFE channel contain the low sounds (bass) that are normally send to a subwoofer.

If you have good quality speakers that are capable are handling bass, then you could try enabling those two options. It is a matter of personal preference whether you like the result. Just try with the options enabled and disabled and compare.