Android In-App Purchase Subscriptions and Different App stores

I am planning on launching an Android app with in-app purchasing to the Google Play store and with Google Play’s in-app purchasing. Wanted to check that no other payment methods are allowed or would get approved by Google.

Secondly - I am planning on making the app available in different Android app stores for download. 3 questions: For a different App Store do I need a different binary and apk to upload and maintain?
Secondly - can I use Google in-app purchase for an app downloaded from another store?
Thirdly - unlike app in google store - I assume I am not constrained by any payment process.

Good planning but you wont find anything negative until and unless you implement it. Best of luck for implementing it and creating your app.