Rhys, Its a good thing to seek advice from others on a forum like this. But sometimes you may get responses you don't anticipate. This might be one.
Before I even looked at the numbers I saw a feature of your design that is almost certain to cause problems - too many rails! Those little short gray hairs on the back of my head stood straight up when I saw that. Please don't do that.
Getting two rails to remain in perfect alignment from installation through operation is hard enough, but three? Extremely difficult. Four? I doubt it.
Very short stroke applications (like die sets) might work acceptably with 4 guides, but any issues like alignment, binding, wear, offset loading, are all just multiplied by stroke length.
I'm sure some amount of the white hair on my head has come from dealing with the issues caused by designers that tried to create similar configurations on other machines. What you're trying to do may be theoretically possible, but the real world will bite you, hard.
Plus - There is an added benefit to going with just two rails, most of your questions about shared loading just go away! I would strongly recommend you find a way to mount the whole thing on two rails. (If I was a little too harsh in my response please accept my apology.)