Don't just focus on smartphone users, add the invite functionality into the standard wallet as well or think of a distribution way that does not lock out potential users. My smartphone just broke for example, although that's a non-issue since it will take quite some time until 2.1 million users have been invited (unless I am right with my suspicion about the system being easy to cheat). However there are many potential users without smartphones who can't participate in the airdrop and I'm still not convinced that the system can't be cheated. MAC/IP addresses and emulated hardware can easily be changed, not to mention Google accounts.

50% premine is huge and a fair airdrop everyone can participate in, aswell as being transparent about where how many coins went is crucial. Telling us how many coins were distributed is not enough at all, who can tell us that you don't keep e.g. 20%? A website with premine distributon status should be minimum, don't hide these (far too incomplete) stats in the app. Every other distribution method, even a shitty giveaway per mail address or bitcointalk user would be more transparent, less easy to game and fair to everyone.

Also two typos in OP: "Fare & transparent" should be "Fair & transparent".

300 coins?! That's ridiculous, three full blocks... Will you pay out smaller miners with less than 300 coins or do you just keep their coins?  Also why would you hide the payments site and how can/why can't users check how many coins they mined so far? This smells like a scam.