I had exactly the same experience - measured/weighed everything, bought cake flour especially, followed the instructions exactly, but cake was rubbery and gummy layer at the bottom. I too used a mixer (but on low, as instructed and kept it to an absolute minimum.) My batter was more liquid than expected. The chemistry for a vegan sponge is so tricky to get right! I'm a pretty experienced vegan baker so it was disappointing but I also know there's variables one cannot always explain so....will probably try it again!

The recipe hasn't been tested with anything else so I can't recommend a substitute. You really can't taste it though. There is only about half a cup in the entire 3 layer cake.

An 8×2 inch pan holds about 6 cups (1.4 litres) and a 9×2 inch pan holds about 8 cups (1.9 litres), so you'd need to scale the recipe up by about 1.3 to keep the sponge layers the same thickness.
You could just leave it the same though and just accept the sponge layers will be thinner (and will cook quicker because they are thinner).
I can't advise on baking times for either option though as I haven't tried it. You'll just have to use your judgment and test with a toothpick or skewer.
Hope that helps!

So pleased you enjoyed it, Josephine!

Oops, a lot stronger*

All ovens are different and the pan you use makes a difference to bake time. This means that with cakes and muffins the times given can never be exact. Just bake until a toothpick inserted comes out clean. As long as the recipe was followed exactly there's no other reason they wouldn't bake through. With regards pans, if you used silicone that can take substantially longer than a metal pan to bake because you don't have the heat you do with metal. I hope that helps!