I have not used multiple dhcp-ranges before. but it seems to me for the problem 
2) When I set the tag for the pxeboot group, it was not honored by the
DHCP. Why?

for the dhcp-range command should it be.
dhcp-range=tag:devbox,10.1.2.1,10.1.2.255,255.255.0.0,12h

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