OK, you have a typo in your "XComLW_Overhaul.ini"

It says

ItemTemplateName="SARKGUN_MERCPLASMA"

instead of

ItemTemplateName="SPARKGUN_MERCPLASMA"

This typo makes building Mercenary MEC Cannon impossible.

Oopsie, it's fixed now, thanks.

Think i know why it's not showing up for. if you have the Lost Towers mission enabled, you never DO Mechanized Warfare, so one of the MEC Cannon's pre-reqs never happens. probably should just remove that. you wouldn't be building one anyways if you weren't using SPARKs in the first place, so it's not a big either way to have that second pre-req (especially when it doesn't even exist if you did Lost Towers).

Well i changed it, hopefully it works. Curiously enough that's what i did for the vanilla version of the mod, the vanilla version of the mod only requires the plasma rifle(i think) research to get the spark cannon, but when making the LW2 version since they had mechanized warfare there for the weapon i figured i would leave it as is.