I cannot speak for it for cycling. I have never heard of it recommended for that. As to the rash...well that’s not proof or disproof of anything.
IV vitamin C infusions are well respected treatments for certain illnesses. The great thing is that vitamin C has never been known to kill anyone. Conventional medicine hospitalizes and/or kills hundreds of thousands per year. IV-C is being used here in a Houston hospital as we speak, to treat Covid-19. Has been used in other hospitals as well. I guess when a doctor is left with a choice between ego and simple, natural, inexpensive treatments, many will choose ego (or rank ignorance). Oh well.
IVC is made from sodium ascorbate and water.
Cathcart’s DIY instructions:
https://www.youtube.com/watch?v=Zgi-7xPrCAg
I tried to look this up but found little that indicated that it was beneficial outside of very specific situations despite having had adherents for decades as both you and I have noted. The belief that certain vitamins, minerals, and nutrients are valuable for health in massive doses that one would never get from naturally obtained sources has always seemed more than a little off base to me. And strangely enough most of the people who I have known who believed such things would extend their lives... died fairly young from various causes. The person who I was closest to who took massive amounts of supplements is still alive but has suffered from dementia since his early 50s.
I do take a multivitamin and a low dose CoQ10 capsule. But the poor outcomes of so many acquaintances have caused me to believe that huge doses of supplements can cause damage to our bodies.