It appears that this can be used underground but not to be outside in the open air! correct? Can it be cut off with a standard tubing cutter

It can be used in both environments. The restriction is not to be used anywhere that it can be damaged intentionally or unintentionally. Yes it can be cut with a standard tubing cutter.

It can be cut with a standard tubing cutter. I've used both a hacksaw and tubing cutter. You have to be careful not to deform the cross section into an oval with the cutter but if you go slowly you'll end up with a clean cut.