Are runways "one-way"?
As noted in the linked question to the question you linked to (this one), runways are almost always bidirectional. In your example, the other runway would be simply numbered 18 higher or lower, for 180 degrees off. Thus, the runways would be 26R and 26L - the "L" and "R" would swap, because you are pointed at them the opposite way. You can see this in the diagram of LAX below (src):



Yes, generally they flip based on prevailing winds, though I can't imagine they flip based on the "whims" of ATC often. Most airports will have a "calm wind runway", which is the preferred runway for operations in calm or near-calm winds.

Due to terrain or similar factors, some runways may be only usable one direction, but both directions will still be numbered, as you will takeoff and land pointing different directions (for example, Lukla airport in Nepal). Alternatively, as Jan Hudec noted, some will operate one-way, such as TNCM, though again, both runways are numbered. Occasionally, the opposite direction is not numbered, though this is rare.