I thought I would report, as I finally got around to attempting to install rooted Android on the 2nd TWRP boot partition. It seems to work fine, with the expected drawback that the modem doesn't work so one can't call. Wifi works fine though, and I can still run Titanium backup, which was my main purpose for having a rooted Android on the phone. So now I have successfully installed Ubuntu Ports over the old rooted Android in partition 4, and have a set up as follows: 1. Android; 2. rooted Android (no cellular); 3. Kde Gemian; 4. Ubuntu Touch. I'm really enjoying exploring Ubuntu touch.
PS I am amused to think that I backed Ubuntu's Kickstarter or Indiegogo convergence phone almost a decade ago, only to have it refunded when they didn't make the millions they'd asked for. But almost a decade on, I've finally got my Ubuntu phone and with an attached Psion-keyboard as well. Happy days!