I'm curious why you think the CF is invaluable for a diamond farm. What kind of diamond farm are you running? It sounds like you're trying to get diamonds through RQs, and since they're one time only investing in a CF seems rather short sighted. Granted the Seed Vault I can see as a super wishing well.
For me a perfect diamond farm consists of 4 GBs: Alcatraz, CdM, CoA, and Zeus. The rest of the buildings consist of one rogue hideout and WW/FoYs. Getting a Seed Vault would be at the bottom of my list of priorities, only to be obtained when everything else is at the proper level. But the CF? I don't see when I'd ever build it.
I'm not running a diamond farm, but at one time I had several. I deleted the account, because I was spending too much time playing, and I've never regretted it.
But, you're missing one important part. The constant events and such, and they give an inconstant, but significant, supply of random rewards, and if you're getting 60 diamonds instead of 20, it adds up pretty quickly.
I also have no use for an Alcatraz, and don't know why you'd need that unless you auto-battle your way through GE. I have limited experience doing that in later stages, so I'm not sure how well it works. But, one thing to remember is, there is a finite amount of diamonds you can get from GE, and the amount of diamonds you get from events and such does not have a hard ceiling. So, after a point, none of those buildings you mention will be of benefit, whereas CF will always increase your diamond production.
Plus, I would advance with my diamond farms, albeit slowly, so I would get new quests from the story lines. The main thing is though, if you do the events, it can have a positive impact on the amount of diamonds you win, especially if it's obscenely high level.