Pre low-carb, I always replaced half of the flour called for with rolled oats when making a crumble crust so I have had really good success making a crumble crust using a combination of almond flour and unsweetened shredded coconut. You will have to experiment with the different artificial sweeteners though, I find them all very different and am personally not a fan of the after taste that a lot of them leave. I usually use Splenda

It would not be super low carb but, for the crust you could try golden flaxmeal. Maybe not 3cups worth. I would go with 2 cups and then add some vanilla (sugar-free) protein powder (maybe 2 scoops) and some powdered stevia or a fake brown sugar... I think Twins makes one?

For the filling, I would use protein powder too. It can REALLY fluff up. In fact, if you look up protein fluff recipes, you can see it whips into an almost mousse like texture after lots and lots and lots of whipping. I would use whipped cream instead of half and half. And of course use a stevia or other sweetener instead of sugar.

I know protein powder in baking sounds odd but I add it to all of my baking and find it makes a good partial (or most) substitution of flour. Most of my baking now includes flaxmeal, coconut flour and vanilla protein powder.