Directing means applying leadership to per A7.53. Spotting is not directing. Among other reasons for thinking so, the spotter can be a MMC. Also per the C9.31 EX, a leader stacked with the spotter does not modify the TH DR, but one stacked with the unit that is, in the words of the example, "firing a SW mortar", does.
As best I read the rule, the spotting unit is not firing the mortar and its DRM do not apply except as noted. In particular, LOS Hindrances from the spotter apply and being pinned applies. Otherwise the DRM are based on the unit firing the weapon, which is to say the one that possesses it, e.g. non-qualified/captured use, CX, etc.
JR