I would advocate the ethical principle that we should effectively take into account the values of non-existent people only to the extent that we can expect them to effectively take our own interests into account.
So, for example, as a recent retiree, I should take into account the preferences of people to be born next year to the extent that I expect them to keep Social Security solvent. On the other hand, I have a much lower obligation to people to be born next century, since I don't expect them to contribute much to me.
As for my obligations to counter-factual people - those flow through counterparts. I have obligations to the Downs-syndrome child next-door because his healthy, but counterfactual, counterpart would have had some obligations to me and to my counterfactual Downs-syndrome counterpart. But neither of us has particularly strong obligations to (nor claims on) some poor peasant in Kerala, because of the length of the chain of counterfactual assumptions and common acquaintances that connect us.
If I understand correctly, you're saying that if you had Down Syndrome and your neighbor were healthy then you would want your neighbor to help you; so therefore in reality you are healthy and help your neighbor who has Down Syndrome; and this constitutes your obligation to them.
Is this correct?
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.