InMobi is the best alternative to admob for mobile display advertising . The reach is truly global alternatively you can use millennial media

Mobile advertising to target users with high precision and through multiple funnels like handsets, OS , handset models , OS version , geo location, demographics, genre , content etc . Define your user segment based on these parameters . Like to target an Affulent user who is stock market savy... Run a campaign targeting iPhone 5 and iPads 2 and above on apps like Bloomberg , financial times, WSJ etc . Thus you can create your targeting segment and target them by running a campaign through good ad network or directly with publishers.

In my opinion html5 works best in case of cross platform integration. Also this is widely used by developers and accepted by google FB and other bigger platforms so it make sense to go for HTML 5 .

-Please get you active users to propagate your app through asking them for reviews in App Store , prompting for Facebook like , share on what's app with friend . With change in AppStore ranking algorithm reviews will help you generate good ranking and place higher in search results.
-If the economics allows gratify them with some offer for referrals ( may excess to any premium feature for a limited time , or points , discounts etc whatever is possible )
- rebuilt you visibility through a campaign where you do 1or burst activities towards weekend starting Thursday evening and sustain with normal activity ( 1/5th of burst) for few days .
- also you must see you LTV per active user and basis that work on activating the dormant users or put this data to use to acquire new user with above mentioned tactics

Ensure that the partnership you are getting into is not interfering into your core development process . However a constructive feedback is always welcomed. While you tend to gain a lot in terms of leveraging the scale and reach, technical expertise of these networks , you should ensure that you are following the vision you have started with .