I got pretty much all the mould out. I let it soak in a water/baking soda solution for a few hours and just scrubbed. There are still some stains but they are not black and simply look like tea stains. I then cleaned the spout out and passed boiling water through it until there was no more particulate coming out in the water. After rinsing thoroughly I put boiling water through and tried it. I tasted no off tastes, but noticed that the pot thickened the water a bit and left a tiny bit of sweetness on the tip of the tongue. It no longer smelt of mould.
I tried brewing some charcoal roasted dong ding in it and it was wonderful. It rounded out the roasting nicely, but didn't take away too much. In fact, it left more roasty toasty flavours that my low (almost underfired) hongni did. Also, it seems there was so much mould in the spout that it slowed down pour time! It went from 15 sec. (pre-clean) to 12-13 sec.
All in all, I'm pretty happy with it and I'm glad I could get rid of the mould. I may post it in the yixing showoff thread to get some more info/translations of the stamps.
Post clean: http://imgur.com/a/hN0Yu
(sorry for terrible photos)