Update:
I removed the bloated battery but still can’t boot from the SSD. It’s probably corrupted. So I booted from the USB stick I had for late 2010 Mac Airs and formatted the main drive (Mac OS Journalled). I tried to restore the OS (using Disk Utilities) from the hidden partition containing the factory disk image in it but still no joy. I ended up installing the original Snow Leopard OS from the USB stick into the main partition and now everything works.