Can not share WiFi from Macbook Pro to other devices via Bluetooth PAN
I ran into the same problem when reverse tethering my Samsung phone (Android 5.1.1) to access the Internet connection on my Macbook Air (El Capitan), and found a successful solution:


Make sure your mobile device is paired via Bluetooth and follow Heiko Haller's
steps for Internet Sharing settings. 
Under System Preferences > Network, select the active Internet connection 
(e.g., USB LAN) and note the IP Address, Subnet Mask and Router numbers. 
Select Bluetooth PAN in the same Network window, click the Advanced button 
in the lower right to open its configuration panel.
Under the TCP/IP tab, select 'Manually' from the Configure IPv4 drop-down 
menu, and copy the information from Step 2 in the three relevant fields.
Select 'Link-local only' next to Configure IPv6. 
Click OK to close the Bluetooth PAN panel, hit Apply in the Network window.
On the mobile device, go to Settings > Bluetooth and click on the cog icon next to
the paired Macbook and turn on 'Internet access'.


It's been a couple of hours and so far, the connection seems stable. 
Edit: An incoming regular call broke the connection, and unpairing and re-pairing the device appear to have restored it. 
Edit2: The IP Address and Router numbers may change each time you connect to the Internet, so be sure to update the Bluetooth PAN settings accordingly.