Interesting... I think it works.. All the letters are there.. I never thought of that.. Very creative
I personally wouldn't. I know the letters are in the right order, but the starting sounds are so different. I see Isla as its own, stand-alone name, totally unrelated to Isabella. Sometimes I do like stand-alone, separate names as nicknames, but usually that's to take a wilder, GP name (Mars, Thor) and help it fit my style more (Marshall, Arthur). Since Isla and Isabella are on a fairly similar plane, I confess it doesn't make as much sense to me that it would function as a nn for Isabella. I too like Isa better. (Granted, all that's my opinion. You can, of course, do whatever you like :) )
I think it would be fine.
I think it could work, in theory. While they have the same letters, they don't share any sounds. I think Isabella is closer, but it's still weird. But I see where you're coming from: Isla and Isabelle are match-y for sisters, and both are amazing names (even if you aren't expecting).
But Isla has no connection to Isabelle.
*If you pronounce Isla as ees-luh (the Spanish prn), I think it would make a lot more sense.
I don't think it would work well. They are totally different sounds. If Isla were pronounced "is-la" and not "eye-la," then I would say it could work.