Is there an ad server that will allow us to integrate sponsor ads into our app?

You won't find anyone worth adding to your team willing to work for equity only, no matter how compelling your product and business is. The realities of the talent market for mobile developers anywhere is such that a developer would be foolish to work only for equity unless they are a cofounder and have double digit equity. Happy to talk about hiring and alternatives to full-time hires.

You are right that the range is wide. You need to figure what are good values to have for your category. Also, you can focus on the trend (is your DAU/MAU increasing vs decreasing after you make changes) even if benchmarking is tough. Unless your app is adding a huge number of users every day (which can skew DAU/MAU), you can trust the ratio as a good indication of how engaged your users are. For games, DAU/MAU of ~20-30% is considered to be pretty good. For social apps, like a messenger app, a successful one would have a DAU/MAU closer to 50%. In general most apps struggle to get to DAU/MAU of 20% or more. Make sure you have the right definition of who is an active user for your app, and get a good sense of what % of users are actually using your app every day. Happy to discuss what is a good benchmark for your specific app depending on what it does.

I developed and published mobile apps as an individual for several years, and only formed a corporation later as things grew and it made sense. As far as Apple's App Store and Google Play are concerned, you can register as an individual developer without having a corporation. I'd be happy to help further over a call if you have any additional questions. Best of luck with your mobile app!

Apps are difficult to fund on IndieGoGo as few are successful, and we rarely take them on as clients. Websites like http://appsfunder.com/ are made for that very reason, but again, difficult to build enough of a following willing to pay top dollar for an app that could very well be free, already existing in the marketplace. A site that is gaining more traction you may want to look into would be http://appsplit.com/. Again, Appsplit Is Crowdfunding For Apps specifically.

I would suggest launching in a foreign app store only (ex: Canada). That will allow you to get more organic users to continue iterating without a big push. I got this idea from Matt Brezina (Founder of Sincerely, previously Xobni) https://clarity.fm/brezina - he's the man when it comes to testing & iterating mobile apps.