As much as it sucks to say this, the Vitamin C isn't helping much. I've been taking 5 g the past few days (though maybe that is not enough time) but the gum bleeding has not improved much (though my skin complexion has improved slightly).
I think the majority of teeth and gum disease boils down to bacteria. The reason this becomes more of a problem in hypothyroidism is probably weakening of the immune system and connective tissues which lets them invade gums more easily.
Oral health rapidly goes down when the endocrine system is compromised. This is probably why bell peppers helped, capsaicin helps kill those suckers.
I've also concluded that it's not a best idea to brush with salt water, just rinse. Brushing with salt seems to aggravate the gums more so I just use a soft brush with some mineral water, and then rinse out with salt water. However, salt water is probably not powerful enough to kill all the bacteria. Maybe hydrogen peroxide would be capable of keeping things under control.
Either way for me the whole bleeding gums started after I developed hormonal issues and such, so I guess someone who is healthy will have no problem fighting off oral bacteria but for those of us who are a bit messed up we better find something to keep it in control until we get better. I don't know about the safety of peroxide, supposedly if you just swish it around for 5 seconds and spit it out it should be enough to kill bacteria but not do any damage to teeth or gums.