When you make your phone a hotspot it’s taking control of the WiFi radio and essentially showing it as off to apps on the phone, which then use 3G/4G to access the internet. This means the Tinker app can’t send the required packets over WiFi that tell the Core which SSID to connect to.
The work around is to either program the WiFi credentials onto the Core over USB or use Tinker on another iPhone/iPad/Android device connected to your phone’s hotspot.
You can also download the SmartConfig java applet from TI to do it with a PC or Mac.
As to your Raspberry Pi question, what sort of data do you want to send and in which direction? Tell us exactly what you want to accomplish in as much detail as possible and we’ll try to help.