Nations X and Y have the following trades:
1. X -> Y resource
2. X -> Y gpt
3. Peace, ROP, MPP, Alliance, Embargo
4. Y -> X resource
5. Y -> X gpt
You have written it as if they are five separate deals but note that if 1 and 2 were part of the same deal, then if the resource (1) is cancelled, then so is the gpt (2).
A. X signs an embargo against Y, what on that list gets canceled, who takes a rep hit?
1 and 3 get cancelled. X takes the hit.
B. A third party blockades or destroys a road between X and Y, what on that list gets canceled, who takes a rep hit?
1 and 3 get cancelled. Both take a hit.
C. X blockades or destroys a road between X and Y, what on that list gets canceled, who takes a rep hit?
What I do know is that you cannot blockade whilst at peace. Cutting a road? I don't know if it's only X that takes the hit or whether it's X and Y.
AIs invariably lose their rep anyway so I never really bother about trying to force them to break a deal just for the sake of them losing their rep. I just try to maintain mine when I want to.
Edit: Also, when pop rushing, do foreign nationals get used up first? (Could I join a few foreign workers to poprush without killing my own people)?
I rarely draft but I do know that you cannot draft foreign nationals. I therefore assume that only your citizens would get used up and so this route is really not what you're after.
ALSO, how is the price of rushing determined? Both for gold and pop.
Coin rushing:
4 gold per shield unless there is nothing in the bin, in which case it is 8 gold per shield.
EG: To rush a knight straight off, 70 shields=560gold. However better options would be either to rush a worker at double cost (10 shields= 80 gold) and the rush the final 60 shields at normal cost (240 gold) for a total of 320 gold.
OR disband a unit first to get some shields in the bin and then not suffer the penalty. For the knight example, if you have a warrior to disband, that will give you 2 shields in the bin. Rushing the other 68 shields will cost 272 gold.
Short rushing is another option. If that town is producing 10 spt, then let it have one turn of production. Then rush the next 50 shields (200g) by rushing a musket but then switch back to the knight production. the town the produces a further 10 shields to complete the knight in two turns. This can be pretty expensive so I tend to use this more when a city is wasting a lot of shields in an overrun. Eg for a 20spt building a knight, I'll rush the 20->30 stage to cut down production from 4 turns to 3 at a cost of just 40 gold.
Pop rushing:
If you have some shields in the bin, each citizen will give you 20 more so:
1-20 shields to complete the build needs 1 citizen
21-40 shields to complete the build needs 2 citizens
etc.
The double cost penalty is still in effect if you rush with no shields in the bin. i.e.
1-20 shields=2 citizens
21-40 shields=4 citizens
etc.
A knight with no shields in the bin would cost 8 pop points!!!
Waiting a turn would cost 4 pop.
Waiting until you have 10 shields in the bin would reduce the cost to three pop.
Also note that you cannot whip more than half the population at any one time so for larger builds you need to do it in stages. In the situation above where you waited for the 10 shields but only had 5 pop points, you couldn't rush the knight straight off as rushing 3 pop needs a town/city with at least 6 pop points. However if you switch the build to a settler, rush the settler for 1 pop point, taking you up to 30 shields. you can now switch back to the knight and rush the final 40 shields using another 2 pop points.
(I presume you know about the associated unhappiness caused by the whip?)
Hope that helps but please ask again if any points are unclear. I'm typing this at 5am!