For example when you are grieving for a loved one, don't you rather want to hear some comforting, soothing half-truths?

I can handle my feeling on their own and I don't need someone to lie to me to comfort myself. Fully accepting reality, allows processing of emotions much better.

2/B) Shouldn't a species with a generally Low Sanity Waterline rather construct something along the lines of lest harmful / most useful Designer Religion (parallel: designer drugs) as opposed to trying to overcome it entirely? What would be the ideal features, goals, deliverables of a proper Designer Religion?

Being conscious about ideology is useful but I don't think it's very useful to think in terms of religion. For Muslims rules about how inheritance works are part of their religion. For Christians that's not true.

Effective Altruism does fulfill some social values of religion. It doesn't need a God to do so, or forbid it's followers to believe in Gods. If you want spiritual experiences there are various practices that don't need any decision to believe in Gods that might even better at providing spiritual experiences than Christian religion.

This deserves a rational analysis but I don't even know where to begin! Is there something like a narcissism test for example?

Of course. Academic psychology attempts to measure a variety of traits.