I've been running some tests with it on and then off and it seems you have a much higher drop rate for items when you've rung the bell. And the description does say it may grant more rewards so I believe it to be true. Not sure on if the gold per kill increased though.

Whilst farming some cash with the bell on I didn't notice any increase in difficulty of the basic enemies or increase of money. I did however notice that they started dropping scrap iron and black gunpowder with surprising frequency.

I havent checked the numbers before ringing, but after doing so the enemies dropped something like 11 Sen and 44 Sen, which could mean they drop 10% more. Just a speculation, though. The difficulty seems to be the same.

How do you Remove that Debuff?

You can use the demon bell after the fire bull mini boss at the woman praying and she will gift you something.

doesnt increase money or exp, felt like enemies had slightly more posture and HP, but i need to test that more

I think this might increase aggressiveness of enemies, it seems to substantially increase drop rates of consumable and upgrade items. if there are changes to enemy posture and hp it's too low to notice.

I've heard that you should be able to ring the bell again to remove the effect. So the difficult increase is a on/off state only.

Seems to drop items more frequently other than that not sure