I don't really think you need the GSE, but if you opt to use it, I don't think it will hurt your beneficials too much. I used it but I can't honestly say it had a benefit. IMO, the C is sufficiently strong by itself.
I can't point to a great explanation of why the Mag Carbonate works. Myhill is vague about it. And I tried Cal/Mag Carbonate but worried about the Calcium, so switched to the Mag.
I don't think it's critical, but it feels very soothing after the C, and it does something to increase peristalsis, not exactly sure how, but it's not subtle. You really feel it. It made me very sleepy as well.
I think the Oxbile would be excellent to add. I took low doses and had no trouble.
I stopped Probiotics for 5 days and that sped the process up a lot. I then resumed them without the previous trouble.
Another observation: I knew I was done with the C when I stopped absorbing it as readily. There was a definite arc to it. Absorption/Tolerance went up quickly, and vice versa.
I peaked at 12 grams.