When the pipeline A (figure 1) and pipeline B (figure 2 below) are in shut in condition, can ATMOS still perform leak detection?

Figure 1: Line A - (Line A: here valves are shut so, ATMOS gets no flow readings from the flow meter)

No problem to detect leaks for Line A. It has been done for Shell, Total and Esso (Exxon) before.

Figure 2. Line B - (Line B: Here a check valve assures that product can only flow one way. If meter is on far side from a leak, it can not function in leak detection, how will this affect leak detection and location?)

For Line B, leak detection will still work based on the reading from the pressure on the leak side. However, leak location estimate will not be as accurate as for Line A. It is still possible to tell which side of the check valve the leak will be. The deterioration in the location estimate will depend on the location of the check valve.