I'm not sure I see the relevance of that remark. In any case, this forum - like so many others - marks all external links as "nofollow", so if you're looking for backlinks from a forum you may as well forget it and do something more useful instead.
I think 500 is more and its over optimization what my personal opinion is and its not going to harm them now because as google bot takes time and no no one knows when google bot will come on which site it will crawl.But once these pages are crawled and if it were from low quality back links the site will gonna suffer.
YES! Google would penalize you for it. In fact, some site owners are actually cleaning up some backlinks as Google seemed to have sense something abnormal with the generation of backlinks, in other words, spamming. Build links naturally - that is to generate few at a time; and create unique content and avoid using much targeted keys to make the contents as natural as well.
500 backlinks back links per day is surly contain duplicate and bad links which directly affect you site ranking.Instead of doing 500 directories on your site do limited and quality work.
It is possible to find 500 links each day but to find 500 links from closely related sites that also have some pagerank would be impossible. You can never have too much of a good thing but if you don't consider quality, you can have too much.
Yes, i agree with you. Over optimization is not good for your site promotion. Because it will treated as spamming. If you want to keep your site with search engine friendly, you need to build back links using withe different link building techniques like forums posting, blog commenting, article submission, social bookmarking etc...