When i think about the setup with the cables... connecting a second board this way would mean that the power would flow completely through the first board until it goes to the second one.
No, it doesn't flow through the entire PCB and its power conveter circuits. It will flow just through the PCB traces between the PCI-E connector and the green power connector. As burnin states, these few cm of traces between those 2 connectors are rated for the current of 2 boards (1 non-overclocked board about 40W, so the traces are rated probably for 80-100 W at 12V, which would mean 6-7 A).
So its the same flow of energy like you would power the board through the green connector directly like you have it when pushing 2 times the wattage in per atx and take one half of it out of the green connector again.
Wouldnt it then be possible to not connect the solid cables to the green connector but instead directly to the atx-incoming-power? Then there are no pcb-parts in between that could brake and the attached cables could draw the power to the other boards. I mean that would be the same like using atx-y-cables isnt it? Of course im not sure if such kind of DIY wouldnt involve a portion of risk.
DHL tells me they want to bring my miners to me tomorrow.