So I just got this email. Well no, I got the email 23 hours ago, but I was too busy reloading PCBWay's site waiting for any messages to arrive.

From PCBWay

Our factory has provided feedback that these inner grooves cannot 
be made because they are too small. Could you please modify the design?
Or do you agree to have our factory process the parts separately? 

Please check the attached file, thank you.

So I'm assuming that when they say "too small" and point to the 4 channels, I can only imagine it's that 1.2mm is too short for them. I was actually expecting them to come back to me saying something along the lines of "sir we can't make a 1mm groove with a 2.5mm bore". 

Not really sure what they mean by "process the parts separately", but this is my chance to inject the upgraded version into the manufacturing pipeline!

I've laid the stack in the order I would've like to see it (not possible if the B-B was 2:1 scale). I was expecting the B-B to look too small to see features at 1:1 but it seems fine actually. I've increased the internal groove to 1.8mm since they never specified exactly how much larger it needed to be, and that conviniently makes the entire block 1" tall. The mass is now 153g.