Are "mp2v" and "mpgv" valid MPEG-2 codec tags in an MOV?
MPEG-2 doesn't seem to be an officially sanctioned codec for the Quicktime container. The official specification has a section related to MPEG-1 in MOV but not MPEG-2. The US Library of Congress also does not have an entry for the subtype MPEG-2 in MOV, like it does for MPEG-1 (just called QTV_MPEG).

So, there's no official word on "allowable alternative tags". One can only examine MPEG-2 MOVs and see whether apps, specifically, Apple apps, play them or not and prepare a survey report. Since each app has its own fiat on whether it plays a file with a certain codec ID or not, that report whould be categorized by app i.e. VLC, Quicktime player. FCP..etc.

As an example, VLC recognizes the following tags for generic MPEG-2 i.e. not IMX or XDCAM:

    A("mpeg"),
    A("mp2v"),
    A("MPEG"),
    A("mpg2"),
    A("MPG2"),
    A("H262"),