How do you get 8000+? Gold spire x 8 gives 2,200. Where are the other 5,800 diamonds coming from?
I made a small error, I personally get near 900-1000 diamonds a week from the spire but thats for 2 worlds not 1 world.
You only calculated the guaranteed 275 diamonds from a gold fellowship, but off course you also get some opportunities for 15, 20, 50, 75 and 125 diamonds, on average you will get about 450 diamonds a week per world from the spire alone consisting of 275 fixed and a 175 average variable component.
8*450 = 3600 diamonds.
The next question is how many chest can you craft together, each crafting chest on each world will on average give 55 diamonds (10 chest give 4x 20% chance for 500 diamonds and 1x 30% for 500 diamonds)
If you can get 100 VV a week on each world then you get on average 8*55= 440 diamonds
If you are lucky you find a ghost in the spire. I think on average you find 1 ghost every 50 days (I am running 2 ghost for a long long time now on my diamond creation city.)
A Ghost will give about 1100 on average every 100 days.
8 world * 2 ghosts * 1100 diamonds / 25 weeks it lives = 704 diamonds a week.
So with determiniation ignoring other diamond factors not named above you can get ~ 3600+440+704=4744 diamonds a week on the us servers.
And if you can craft more than 100vv per week per world you can increase that number by a bit more.
So my 8000 was an error since I play 2 worlds not 1 world, but half that should not be to difficult with time and determination