I've kept a clod roast going 15 hours & pork shoulders almost that much......low heat isn't a problem.......
Higher, like your chicken is a different animal..........
The coals peak in heat not too, too long after you dump & with no action, there's nowhere to go but down in temp.................
If you want the temp to stay up at the starting temp for a longer period, anticipating the drop is key........
In your shoes, I would have lit 1/2 chimney not too long after the initial dump & maybe added that in @ 2/3's to 3/4 thru the 1st set of cooking.............
Using unlit is OK, but how fast they lite & come to temp is less sure than pre-lit.....
Other way is like stated above: Sort of modified "Minion Method", but with a thicker packing of coals in a chain / fuse fashion..................