They do accept non-profit organizations as well, but I'm going to guess you're not officially that either.
The most common payment providers for NSFW sites is Epoch or CCbill I know some fairly small sites that use them and have mostly no issues with them. However I have heard their customer service isn't the best. Unfortunately payment in the NSFW field is in need of a shake up.
Down votes aren't a great feedback tool. There might be countless reasons why people don't realize why their post came off wrong and in those cases it's better to tell them with words why you have negative feelings towards their post. Similarly if you just don't think the picture they drew or captured lives up to your expectations it would be better to lead them in the right direction by telling them they can improve in lighting or so on. I genuinely think most people want to improve.
I definitely think thumbs down has it's place. It was crucial for the infamous EA's "pride and accomplishment" comment, but it's the lesser used button and the most easily abused button of the two. The down vote has for example been abused by groups on other platforms to extort posters. Lastly it's not that important in determining what gets popular. If you don't vote you already did your part.