Your current setup with an iPhone 6s and a Galaxy Tab is probably giving you a pretty good idea about the advantages and disadvantages of both platforms. But regarding your queries:
1. No, there is no extensive support network in place for Android like what you can get from the chain of Apple Stores. If you do have a problem with your Android device there are in-store repair services at a lot of computer stores.
2. As far as what apps you have on your iPhone, while there are often be similar apps available in Google Play, whatever you bought from the Apple Store won't be transferable to Google Play. The two are completely separate, there is no compatibility nor exchangeable items.
3. If you and your closer friends rely exclusively on things like iMessage or FaceTime, once you switch completely to Android you'll most likely be excluding yourself from the mix on a frequent basis. As an example, while iMessage can optionally send SMS texts to an Android user, by default iMessages can only be exchanged between iMessage users so unless those friends also take the extra effort to include you, there is no iMessage app for Android (and Apple has given no indication it will ever allow it). Contrarily, SMS being an open standard, when you're using Android you'll be able to communicate freely with anyone on any platform. In a similar way there's FaceTime, it's a video chat environment limited to Apple users only. Google Hangouts on the other hand, allows you to video chat with Android and/or iOS users.
4. Can't comment about battery life, don't own neither phone to even make a guess. And really it's such a subjective topic directly related to how you use your phone and how it's been configured -- for any model one owner will say battery life sucks but another will say it's fine.