But onwards plans are relevant because CBP officers evaluate things like intended length of stay / lack of stay, whether you have onwards/return tickets, etc. After all this is declared on the blue form. They might be more lenient if it's clear you're in transit vs trying to visit the US.

Without evidence to the contrary I wouldn't assume that they'll return someone who is inadmissible as opposed to escorting them to the flight they already have tickets for. Your citizenship and residency of third countries might also play a part in this, especially if the onward flight is to your home country.

Finally, doesn't IAH (or possibly MIA) have simpler arrangements for international transit? Still involving CBP but no luggage handling?