Why won’t the pudding thicken up

A very common type of pudding is a pudding made from liquid thickened with starch. It is usually made with milk, but you can use other liquids as you see fit. Fruit juice will probably work best. For starch, use plain potato, corn or wheat starch. The "absolutely no fat" condition is very detrimental to the taste. A trick to make it taste richer would be to include some whey powder. Any sweetener will work - sugar, baking sweeteners like honey or agave syrup, or artificial sweeteners (keep in mind the max temp for artificials). You can enhance the flavor wit the usual suspects - vanila, cinnamon and other spices - and/or serve with syrup poured over. Adding food coloring is also a good option, as it affects the perceived taste too.

To prepare the pudding, use a base ratio of 1 part starch to 10 parts liquid (you can change that later, depending on your preference for firmness). Mix the dry ingredients (sugar, cinnamon) with the starch in a small bowl. Stir in some cold liquid, around 1 tbsp per 25g starch, until you have a slurry. Bring the remaining liquid to a boil. Remove from the heat and quickly stir in the slurry. Return to the (reduced) heat and let simmer for some minutes, stirring contantly. Stir in the liquid ingredients (e. g. honey, molten caramel). Pour into the form(s) and leave it to cool, at best overnight. It will set and should be firm enough to slice. Alternatively, it can be eaten warm (and still soft) from bowls.

You mention Jello. I don't consider gelatine jellies to be a "pudding", but maybe this is a language problem on my part. You can make a gelatine "pudding" with powdered gelatine, just follow the instructions on the package. Gelatine itself is pure protein and does not contain fat.

It is also possible to use modern thickening agents to thicken a juice or another liquid to a more or less pudding-like consistency, but this is unusual in home cooking. If you are interested, look up guar gum or xanthan gum. There is a free e-book oh hydrocolloids often linked here on SA, but it probably goes much beyond what you need. Still, you can search for it if interested.

It is difficult to give an authoritative answer because who knows what a "typical" pie might be like. It might be different for every person you talk to. I will therefore answer just for myself.

All of the made-from-scratch pudding pies that I have made have been very similar- a lot of sugar and fat and some starch to make the gel. Usually recipes also call for egg yolks which also help with binding but make the pudding a little more custardy instead of just a gel.

The texture should be sliceable so that the sliced edges sag but don't flow. I have had pies that have to be scooped out which, while perfectly delicious, isn't as nice a presentation. On the other side of the spectrum is what you have- rubber- which is a great presentation when cutting but not much fun to eat.

Gradually adding your corn starch slurry is always a good idea. Often the starch is mixed with the sugar to make sure it is evenly dispersed and then added to the liquid. As long as you didn't have clumps your technique here was fine.

I don't think your expectations of a banana pudding pie or your technique are wrong. What I think is slightly wrong is your expectations for this recipe. It's "wrong" to call something a "take on a southern classic" that uses soy milk and organic margarine. The milk fat and egg yolks in typical recipes do a lot to make a smooth, custardy pudding. Doing this with different fats and soy milk could be good but I would expect it to be different.

The bananas don't have to be precooked- that would be horrible, I haven't used earth balance but if it behaves like normal margarine then your crust sounds like it is pretty normal. The number of bananas also sounds reasonable. You want a good layer but not so much that the pudding can't work in to bind it together.

Reducing the amount of corn starch a little should make the filling softer and more to your liking. You could try adding a little more fat which will also soften the pie. In the end though, I wouldn't expect a typical texture of a pie that is made completely of non-typical ingredients.