At first, I thought it was just Omen2 and Underhood, but after trying combinations of RecountThreat and SmartyCatManaBar, I can safely say it's the libraries.
An immediate example of this is to load Omen2 and UnderHood together, the mana bar will only show up after either the first install, or after tinkering with options in UnderHood. After the first UI reload or on the next log in, the mana bar will disappear, and unless the profile is reloaded, or said options are tinkered with to cause LibDruidMana-1.0 to 'initialise', it'll stay gone.
It seems that somehow LibThreat-2.0 is forcing LibDruidMana-1.0 into dormancy. Exactly how this is happening... well, I don't know. It'll take a brighter mind than mine to figure it out, or at the very least someone who's familiar with the code.
But for those who're wondering why their Druid mana might be missing? This is the answer.
You actually see a small bar next to your mana bar, that persists into feral forms? I think that this is something where it wouldn't be missed if it wasn't seen to begin with, so I'm just asking to make sure.
And you base that on... ?
But I did test SmartyCatManaBar, and there were intermittent issues with that when I had Threat-2.0 based stuff (RecountThreat/Omen2) loaded too.
I admit that it might be an issue with RecountThreat/Omen2 and LibDruidMana-1.0 though, as opposed to Threat-2.0 alone.
But as I said in the original post, I did test combinations of those mods, and issues did occur. There seems to be something incompatible with those mods, in general. And the only linking elements I could see between those mods were the libs...
It was only a bug report, anyway and all I was doing was reporting my findings. If that's frowned on and everyone wants to jump to the defense of their favourite mods whenever it happens, then I won't do it again.