I'm looking for an email SignUp / SignIn based blockchain wallet which provides API for integration into web dapps and native mobile dapps (Android & iOS). During payment routine in dapps, this API should be able to initiate an OAuth2 / OAuth style Email Authentication routine. Once logged in to wallet, the wallet API should provide option to transact the smart contract functions like metamask. Bonus points for other functionalities like SignUp (register) new users and add cryptos in the API interface itself.
Have a look at: https://www.portis.io
I think this is the closest you'll get to an answer to your question.
It pretty much complies with most of your requirements with regards to features.