Yes, screws were pretty tight, but they are made from quite hard metal, felt really solid (at least on my case). I would be more careful tightening them back up, because the thread is very small.
No, 1070 didn't fit through the holes at the bottom, cooler shroud was too thick (have there been 2mm-3mm extra space - it would have been fine). Also, PCI slot and motherboard itself was in the way. Maybe it is possible to somehow slide in the 1070 first and then the motherboard afterwards, but at the time unscrewing bottom plate seemed like only option.
As we figured out, there have been different revisons of the OSMI, so maybe spaces and clearances may differ slightly.
Dont really understand about the countersunk holes cluster. Have a picture of it?