With the diamond stones, I have not had any experience with the backs wearing at all. They don't really move against anything or take much unsupported pressure so I don't think it is an issue. The diamond part is what wears out long before the backs show that they've even been used. I also use the Moonflex. I also have one of the SVST diamond files which has much more tightly packed "grit" than the moonflex and I have not been as happy with it. I think that the spacing on the moonflex faces gives the debris some space to move away from the active cutting that improves overall performance.
On the file guide, the SVST is metal, it does not flex, it is accurate and easy to use. The Beast Pro Edge also looks nice and is also metal, stable and accurate. Both have plates that can be added so that you file various side edge angles. I don't imagine you could go wrong with either one.
In terms of grit, I think of the 100 and 200 as coarse, the 400 as medium and the 600 and up as fine. I have all 4 of these as well as one that I think is 1000, A coarse, a medium and a fine should be a good set for most needs. If you were just picking 2, I'd probably go with 200 and 400 if you have a metal file and 100 and 400 if you don't,