Answers:
1) That was 20 years ago and everyone would have moved on by now. Call Ted Hall I'm sure he has time for students or someone there can help. He has a successful business so the users must like them
2)Please publish some images to ensure we are on the same page
3) An "I" beam consists of flanges and a web. The web transfers shear between the flanges, the web is the shear structure. We make I beams to improve the structural efficiency of the material. In a twin gantry design as I think you are designing you are using two tubes as the flanges but you do not have a web. I think you expect that two widely spaced tubes are stiffer then one large central tube. This is incorrect thinking. If you do the correct math this will become apparent
Mind experiment : You have two parallel beams well supported at the ends (your twin gantry). There is a stick in the middle that slips in between. You push on the lead beam, how much load gets transferred to the lagging beam via the stick? If the lead beam is very stiff not much, so the lagging beam is redundant. So in the twin beam approach you eventually have to have two beams the size of the correct beam. The twin gantry is actually evaluated in Bambergs thesis and rejected for being less stiff. I rest this issue here.
4) Speak to HAAS and ask them the stiffness of their minimill. HAAS is a philanthropic organisation with great student interest. The stiffness number will scare you I think... Plus look up accuracy and repeatability they are different.
I think you need to define what this machines real reason to exist is. A HAAS is a venerable machine to emulate but so far I can't see you getting there. A machine has to have a strong reason to exist otherwise it will rust in a corner, if in fact it gets built. You need to expand the vision. Companies and people are happy to help and money is not the issue, there is more money in the world now then there ever has been. For instance why do you want to cut tool steel? Make the machine out of carbon fibre? do you realise that by the time you are a good engineer, mills maybe dinosaurs? taken over by 3D printers? Companies like HAAS are looking for the next stuff not the stuff that was done 20 years ago. If you where tasked to take a mill to the moon to cut some moonium would you make it out of 2T of steel? What does your sponsor want? Think beyond your thinking... that opens doors
I'm not invalidating your current trajectory I think there's an opportunity here bigger than you think...
There are a million reasons to halt this project, you only need one really good reason to have it happen
Peter