The design is coming along. I'll try to help with whatever I can.

1. One thing to remember that can be difficult to see is that when using the R&P fixture on a carriage, you cannot use the two outer holes to attach the carriage to the frame. The pivot bolt and spring bolt from the R&P intersect and obstruct where the through bolts would normally go. I guess that you could tap the carriage or shorten the length of the R&P bolts, but I haven't done that yet. Of course, I haven't cut anything yet, so all is left to be discovered.

2. This is why I went with 3030 for my long axis. And it is why I have a low profile bed. I made my gantry create more of the height clearance that I desired. If I were you, I would simply plug in whatever the freestanding height of your long axis rails into the deflection calculator and call that your worst case scenario. I used 10.5" with a 75 lb end of beam load on the 15 dimension of 1530 and ended up with 0.006 in of deflection. Now, you will have two beams taking this load at a fixed distance from each other. Plus, you have designed some in some gussets that effectively shorten the beam length. If I use 7.5", the deflection drops to 0.002 in.

In this example, I used a 75 lb load. This load would be entirely generated by cutting forces on the router (plus momentum from the shorter axis). I estimated 75 lbs because of my experience using routers by hand. If I were to experience a 75 lbs force by hand, I would have to move the router slower through the material.

I suggest that you come up with a way to estimate the amount of force that you are estimating you will see and make your calculations from there.

3. I have a NEMA 23 setup, so I can't offer any advice here.

4. I am going to mount my router so that the bottom of the collet nut is flush with the bottom of the 1530. I see this as the maximum depth the router can plunge into the material simply because my G-code software can only take in to account the size of the end mill and nothing else. If I want to mill deeper material, I just have to get a longer end mill. My 8" of use-able cutting volume pre-spoil board was selected so that I could mill half of that without any worries. This way, I could easily mill 4" stockall the way through.