From my SEO/SEM expert:
1) As long as their site is being indexed and they have no script in the link to you, 50 links is good. Plus if they are directories and you are submitting to the relevant categories that is better then just a link on a home page of a domain that isn't relevant.
2) It is better to mix the links into your domain. Better to have links to your domain and to the inner pages.
If you have a spreadsheet that you use to keep a list of directories you submit to (which I recommend) then go through that list and check all the IP's. Save yourself some time, find directories on different class C IP's.
3) Not sure I understand that one. The more links you get, the better. I wouldn't limit myself to just worldsex. Spread it out to the big guys and the smaller. Just check and make sure the smaller sites are indexed. Remember that if they link to you with any type of tracking, you probably aren't getting anything from the link.
For more tips I suggest you check out the
Adult SEO Blog