As far as I can tell, it has nothing to do with the mipmap/half_scale functionality, but does change based on the width of the destination texture (and the destination texture is half the width if half_scale is set). The comment that was there (which dates back to the initial megacommit) seems to not have accounted for the width aspect; it was first used as an actual stride in bbbe898839467c312c31456334540c20fedc7be3 (the first commit that used it at all).

Implicit conversion operators and enums was removed for parity with std::format (https://github.com/fmtlib/fmt/commit/fce74caa15b24cbc0fcafe4706692cb06cb0b815).

Mainly, the improvements are passing in the current command ID so that e.g. stage numbers can be directly included, instead of saying 0/1 or even/odd.

It stores both the konst selection value for alpha and color channels (for two tev stages per ksel), and half of a swap table row (there are 4 total swap tables, which can be used for swizzling the rasterized color and the texture color, and indices selecting which tables to use are stored per tev stage in the alpha combiner).  Since these are indexed very differently, the old code was hard to follow.