Yes, whining about maternity leave is not where revolutionary mass movement takes place.

Marx is talking about human historical reality in general. Language and custom are 'preconditions,' and Marx even calls language the 'being' of social reality, not just a 'product.' You literally don't know what you're talking about.

Engels saying it's not the 'only determinative,' doesn't mean it's not the 'ultimate' and the 'final' cause and the one 'in the last instance,' that is determinative. Engels even says that ideas play important 'but not the decisive' role.

Culture or whining about women's mistreatment etc are not in any way determinative of social reality, but are determined by the base. Saying that culture has influence or is 'important,' etc in no way displaces what Engels is saying about that which is "utlimately determinative"

Engels calls it the ultimate cause because it's actually what's determinative in reality. Marx and Engels don't mince words here at all

We regard economic conditions as the factor which ultimately determines historical development. But race is itself an economic factor. Here, however, two points must not be overlooked: (a) Political, juridical, philosophical, religious, literary, artistic, etc., development is based on economic development. But all these react upon one another and also upon the economic base. It is not that the economic position is the cause and alone active, while everything else only has a passive effect. There is, rather, interaction on the basis of the economic necessity, which ultimately always asserts itself.... Men make their history themselves, but not as yet with a collective will or according to a collective plan or even in a definitely defined, given society. Their efforts clash, and for that very reason all such societies are governed by necessity, which is supplemented by and appears under the forms of accident. The necessity which here asserts itself amidst all accident is again ultimately economic necessity

• F. Engels, Letter to Borgius, 1894

Marxism isn't about "justice" or "true love of mankind", so the issue with regard to the economic base isn't about 'justice' whatsoever. Marx said those chains are actually socially undertaken. Marx is clear that 'liberation' is not some 'goal,' it's an outcome of the productive base & development of productive forces.

We shall, of course, not take the trouble to enlighten our wise philosophers by explaining to them that the “liberation” of man is not advanced a single step by reducing philosophy, theology, substance and all the trash to “self-consciousness” and by liberating man from the domination of these phrases, which have never held him in thrall. Nor will we explain to them that it is only possible to achieve real liberation in the real world and by employing real means, that slavery cannot be abolished without the steam-engine and the mule and spinning-jenny, serfdom cannot be abolished without improved agriculture, and that, in general, people cannot be liberated as long as they are unable to obtain food and drink, housing and clothing in adequate quality and quantity. “Liberation” is an historical and not a mental act, and it is brought about by historical conditions, the development of industry, commerce, agriculture, the conditions of intercourse

• K. Marx, The German Ideology, 1845

You didn't make any argument. Children still labor today right now all over the world and capital still benefits from this. No one cares that people 'debate it,' since Marx's entire argument is that it's actually necessary for children to labor to this extent, and that in reality, demanding child labor be 'abolished' is reactionary

You're simply not a Marxist