Maps require an internet connection, though there is the option of caching them, which is what UAV Toolbox does.
I can't condemn anyone for installing apps on the ST-16 (as I sell one!), but the question really is, if you have to jump through hoops to make the Play Store work (which involves fiddling with services the main flight mode app uses), how certain are you that you've not introduced a conflict that might surface during flight? The main flight app has been tested quite thoroughly on the specific Android build that the ST-16 ships with. If you change that build to make the Play Store work, then you're flying on an untested system. Of course there's a certain amount of FUD (Fear, uncertainty, doubt) in that, but caution here seems reasonable.
As regards outside sources for software, I can't beat Yuneec up for not developing every last function that owners want - it's very clear that there is an absolutely vast range of uses for a drone, and it's just not commercially viable for a company to provide every last feature needed for those uses. The size of the team necessary to do that would be enormous - there are multi billion dollar internet companies that have smaller teams. Nor is it particularly healthy to only have one team developing everything. So a strong ecosystem is a good thing - it provides for niche uses, and different takes on the same idea as well as steady evolution of features.
That said, it makes sense to distinguish between safety critical software and support software. To date, I've deliberately avoided writing anything that provides real time control of the Typhoon because I can't guarantee that it would be safe in all circumstances. It may be that the SDK could provide a real time component that can be safely used alongside third party tools - in which case, I'd be more than happy to develop some interesting things.
It's worth remembering that GoPro will have tested the Karma quite thoroughly, and they still missed a critical safety flaw - so I'm not arrogant enough to think I can magically write flawless software without the support of a well tested core. That's where an SDK can really help.