Hi,
You’ve certainly done your homework there 
What happens if you re-run the preseed script manually ? (This normally runs only once during first boot and reads /boot/preseed.cfg to configure network settings) To do this:
sudo /usr/bin/preseed
Ethernet needs to be unplugged for Wifi to be configured so you’re best doing this at the local console. (Exit from Kodi and press ESC at the OSMC splash screen to log in)
After running preseed see if wifi is working, if not check the system journal towards the end for any output from preseed:
sudo journalctl
Also you could try the connman commands listed in the following post to see if you can manually configure the network, and if not let us know at what point you notice an error:
Thanks.