Can I substitute cornstarch for flour in a recipe?

Some pastry cream recipes call for a mixture of flour and cornstarch, so you can try with just flour, but it may change the texture and possibly the flavor as well.

Or you could try making a custard mousse as opposed to the pastry cream. You can probably use the same ratio of eggs:cream:sugar as the recipe your using calls for. But put it together differently. Use heavy cream instead of milk or half and half and whip it to stiff peaks. Separate the eggs and beat the whites with some of the sugar to stiff peaks. And combine the egg yolks with the rest of the sugar and vanilla extract (or whatever flavoring you use) and cook in a double boiler until thickened. Fold it all together. It's the same flavors, just a fluffier, creamier cream filling. Personally, I'd do this rather than use flour because I know it would work. I won't swear that the flour method would work.

The cream pie will turn out to be more cake like and possibly very gross... if you have some eggs and cream or perhaps milk you can make a custard which would give you a better product at the end.

To make a custard take your pie filling take 4 or 5 egg yolks and whisk it with the Sugar in your recipe... boil some milk or cream and add it to your egg and Sugar mixture slowly... DO NOT SCRAMBLE THE EGGS. mix all the cream and the rest of your pie filling together... and bake it in a water bath. at 325 until it sets like a pudding. cool completely in the icebox and serve...

I would not recommend it can be substituted for thickening sauces and gravies but as a filling I would not chance it better to make a custard and use instead

Yes, should be fine. It's just for thickening.

Yes but not recommended.Will give "obviously" a more floury taste.

only if you are using it to thicken.