Yes. It is too big a space to ignore. It’s hard to say about download numbers as each marketplace has its own ranking system. But if you have an app that does not rely on Google Play Services API or it’s not too much hassle to get around that issue it’s well worth doing.
Yes I am very positive. You only have to see where Google Play isn’t available. China is one of those places and 1.36 Billion people live there and they like Android smartphones 
(I have no idea why Google Play is not available in China, but I’m assuming it will be in the future though.)
If it’s not a huge hassle and you don’t rely on Google Play Services or have alternatives then it’s silly not to try releasing to alternative marketplaces. You can also be in charge of your own IAB as well which means nearly 100% revenue.
Hi Amorgos, I would suggest you take a look at Lotaris (Software Licensing Solutions for Mobile Apps | Lotaris). It seems like they currently have a store independent in-app purchasing solution which allows your users to pay via Credit Card/PayPal/Mobile Operator Billing. They also have plans to integrate with the checkout systems of the various stores like Play/Amazon/Samsung to prevent you from having to learn the APIs of these various stores.