From what I've shown you, can you make a determination on if this is indeed underage nudity?
I have never seen these characters before. If this was something like Pokemon that my kids watch then yes I can say they are of those characters. The counterparts that you sent along with your images are difficult to determine their age from just looking at them, but I can guess that they are teenagers. Please don't take this the wrong way but your work really does not compliment the anime version as you may have intended. There is a likeness to the anime versions but they do not appear as the actual examples you sent. Perhaps a more mature version of them but if I did not know from the examples, I would not be able to determine the age from looking at your work. To answer your question clearly, no your images do not appear as under aged nudity as you stated. If it was your intention to fully copy the anime version then it would be.
Why does my renders appear to meet their violation standards and not the ones in the links I provided?
I was not able to see the links until I created an account with the site. I am not sure how the site operates but the examples you provided appear to be similar in nature. Your images contain the same level of nudity that the examples do. Some of the examples contain sexual situations and visual genetilia which is worst. My immediate recommendation to you is to report those images to their webmaster. If you are asking me for a comparison, I cannot officially provide you one, but the examples do cross an offensive level. I would like to point out that in order for me to view these images, I have to acknowledge the option of viewing adult images on the site. This is a common practice on most sites. I would think that the webmaster of the site is not aware of all the images in your examples.
Was it fair to remove my images and place them in violation?
Again I cannot comment on how sites like this operate. The owners or operators can do pretty much whatever they like with respect to members. As you are a paying customer, you do have the right to report the issue to consumer services like the Better Business Bureau.
How often do you guys go after things like this?
I really cannot share that information but we do respond to all claims. I can assure you that if your images were indeed illegal, regardless of our relationship or your immediate security clearance here, you would be charged. In cases of that nature, Canadian law is very strict and will seek prosecution even if the issue is minor. I cannot speak for the state of California where most of the porn industry in North America comes from. We get several questionable cases like this and in most instances a warning is issued to the webmaster or domain owner. Your work does not fall under that category so consider yourself lucky.
What do you recommend I do in the future?
Well Rob, put some clothes on your images. If you really want to continue making nude or pinup art then put a disclaimer with the image stating what their intended ages are. I think your work is very good but you should concentrate more on things like video games or mature characters. Making art based on questionable characters might only get you in trouble and I would not mind arresting you, if not just for the fun of doing so.