1) I've browsed sites for third-party-organized (i.e. VTG) singles cruises, and while I noticed that they attract broad ranges of singles, does that include an appreciable percentage of people in their 20s? Or are the structured cruise events generally for other age groups?
These type of specifics will have to be discussed with the Singles Dept of the companies you're dealing with. They can give guidance on which itineraries and sailings are geared toward different demographics. They certainly can't discriminate based on age so any age group can participate. They can, based on experience give you expectations for different sailings. Short sailings generally draw younger passengers and RCI and Carnival draw younger passengers than other lines. These type of groups are set up to fit you to the cruise that will best fit your needs.
On the other questions, I'm not going to be of much help, I've been married over 25 years and am clueless on singles issues.
Good luck, Neil