Quick history: Battery wasn't staying charged. Battery is fine, alternator is fine, found that the post/wire at the false bulkhead was corroding and had a bad connection (14.4 going in and about 12.4 passing through to the battery). Even still, the car would usually struggle to start and then run perfectly fine.

Cleaned and brushed the connectors. Now there is between 13.5 and 14.2 coming in and passing through.

Car starts perfectly, but there's an initial dip in idle, which makes it nearly die, but then it's fine, however, now if I rev and then release, things go dim, the engine stalls, and sometimes it throws a "battery not charging" error. Seems to run ok unless the idle dips. Never had a problem with this before - again, it was just having a problem charging the battery before, because not enough voltage was getting back to the battery for charging.

Any ideas?

 

You should connect a voltmeter to the bulkhead post and see what happens to the (alternator) voltage at the time when, after revs, things dim and engine stalls (let us know).

In any case, you should inspect/clean all connections in the main fuses box in the boot, in front of the spare tyre. Then, check whether the battery cables get warm/hot after the engine start and while it is idling. The crimps on these cables tend to develop resistance inside and cause voltage drop, even though the cables may otherwise look perfect. Due to this problem, I replaced both of my battery cables with heavy gauge, 100% copper, welding cables, new battery terminals (soldered to cables) and cable lugs.

 

Maybe a bad voltage regulator? I'd second the voltmeter testing at bulkhead while revving/dimming. Mine started to do similar things at idle where it'd sort of dip in voltage until the engine was revved, where it would surge and stabilize afterwards. Replacing the voltage regulator fixed that. Voltage would hold extremely steady, at all RPM's. I may actually need to replace it again, getting a bit of flickering at idle recently...

If you can remove the alternator yourself, you can replace the vreg. I don't know if it's possible to replace in situ, but I think it seemed to be more work than to just pull the alt, if possible at all. If you decide to get a new vreg, here's the part numbers from my notes:
voltage reg: IN9218HD (OE: 126000-2640)
I think the OE number was the Denso unit, and that's what I would lean towards. The IN*** one died pretty badly. If you pull the alt and do that work, you should also inspect your brushes and rotor contacts, in case either of those are worn as well.

You can also get your alternator tested by many parts shops, maybe worth doing if you're not sure about its condition.