yes it is possible, doing the odd jam with a song here and there should be fine but if you are playing a full set then the assumption would be that you are working and you can't prove otherwise
depends if one is here long term of just a few years. Assimilation takes time and many miss the goods things from back home. In my opinion it's all about balance and Thailand is perfect for that as it offers a variety of life styles and you can enjoy a hybrid of Western and Thai here
Out of those 3 locations i would spend less on Phuket and more time at Krabi. 2 days at Phiphi is about right, any longer can get pretfy repeitive and you'll be sick of the backpackers