Yes, I would say any herbivorous or omnivorous turtle would eat it for now, while it is small and before it gets extremely thorny.
I would use standard potting mix.... aka my recipe.
Throw in some veggie formula osmocote to make it grow extra fast.
Since it is a huge lily, use a huge pot... if you want huge results. If you can find some
completely composted manure, that would be great too.
Or just plain bagged compost.
Or plant it bare root at the bottom of the mud pond and set it free, then you'll get 'native habitat' results.
Contrary to popular belief, my Vic grew VERY fast at a pretty deep placement, at least 30" deep in the big huge front display pond at work. Planted bare root in the mulm.
Water never was hotter than 80* at that depth.
What really screwed it up, was our famous east end Long Island Winds. Every time the pads got bigger than 36", we'd get a wind storm and they would get shredded.
The leaves are constructed with lots of air pockets, like packing peanuts, making them very lightweight and easily blown over....and shredded in high winds, like an umbrella.
Maybe I should try one again, this time in the lotus pond,
(I'd call it: WAR OF THE POND TITANS: LOTUS VS. VICTORIA)
which is recessed into the ground about 3', which helps buffer the winds a bit.