Here is a first attempt. too many nested IFs for my taste but I haven't seen a way around them yet.
A2, A3, etc. your todos.
In column C you need to enter today's date. SGIII has an automator service that will insert today's date. It shouldn't be hard to find. This is the only way I know to do a time/date stamp that is persistent.
D2=IF(B2,"complete",IF(E2,C2,IF(F2,C2+7,IF(G2,C2+14,IF(LEN(H2)>0,H2+14,IF(LEN(I2 )>0,I2+7,"choose a due date"))))))
Highlighting rule for D2 is Numbers> less than $A$1 red fill.
Hope this works for you.
I think I am getting better at this formula writing. I click and let Numbers do most. here is what the formula says:
IF B is checked print "complete" as due date otherwise;
IF E is checked print C as due date otherwise;
IF F is checked print C plus 7 as due date otherwise;
IF G is checked print C plus 14 as due date otherwise;
IF H has an entry print H plus 14 as due date otherwise;
IF I has an entry print I plus 7 as due date otherwise;
Print "Choose a due date" as due date.
I think that is the most nested IFs I have ever used. Good luck!