"fruitlessly alienate people to no purpose and miss the point. "
well it is always good to alienate some people, the better to talk to the others.
Marx did not pursue the critique of religion, because he thought that it was complete, i.e. Feuerbach had done it already, it was time to move on to the critique of civil society. It did not mean that he thought there was anything in it. But as against Feuerbach he did not think that you could shift people's alienated belief systems except by changing their social conditions.
People misunderstand the 'heart of a heartless world' schtick. The 'heart of a heartless world' is no heart at all, obviously, the mirage of a heart, a con, or a desire that arises out of absence. Don't forget the first part, the 'opium of the people'. Marx was no 'sixties druggy. He thought opium was very bad indeed. The image he had in mind was not Wm Burroughs but the opium that the British Empire had used to get the Chinese addicted, so they would be forced to give up their tea.
In today's circumstances, which is something like a slip backwards from the high point of Enlightenment rationality, I can understand the point that the critics of religion are sometimes worse than religion itself. I mean that the Nietzsche/Kojeve/Sartre humanism is a disenchantment with humanity that strips out exactly that which is best in Hegelian Geist, the active, subjective side.
But if anyone wants to make a Marxist defence of religion they should bear in mind that, like a good Hegelian, Marx would think Protestantism superior to catholicism, and catholicism superior to Judaism, and all of them superior to Islam, which is plainly a descent into mumbo-jumbo, and all organised religions superior to new age beliefs, with the worship of the Earth mother Gaia at the bottom of any list he would be likely to draw up.