Can I use my Sterling-LWB5+ M2 card with a single antenna instead of using antenna diversity?

Yes, that is possible.

A special firmware has been created for that purpose.
Search for the correct firmware version in the release folder for the Sterling-LWB5+ in Github.

The software woill be named something like this
and reflects it usage with the mention of "m2" and "sa" - single antenna.

The active antenna port is ANT0.