Reconstruct a zigzagified matrix
Jelly, 18 13 bytes

pS€żị"¥pỤỤị⁵s


Takes number of rows, number of columns and a flat list as separate command-line arguments.

My code is almost identical to the one in the twin challenge. The only differences are an additional Ụ (which inverts the permutation of the indices) and an s (to split the output into a 2D array).

Try it online!