Hi, voner.
Sorry, I don't know of a good writeup on methylation and gene expression and which comes first. I think this subject is more or less at the cutting edge of research at the moment.
I don't think the gene expression researchers have considered up to now that there might be a partial block in the methylation cycle at the biochemical level. The issue of the partial block in the methylation cycle was (I think) discovered only in 2004 by S. Jill James et al. in autism research, and it is yet to be fully realized and understood by the research community.
I think this is sort of analogous to the situation with creatinine in urine testing. In the past, it has been common to divide the concentrations of other substances in the urine by the concentration of creatinine, because the latter was considered to be more or less invariant, so that it could be used to normalize the levels of the others, in order to compensate for differing amounts of water diluting the urine, depending on each person's water balance at the time of testing. However, in ME/CFS and in autism, because of the methylation deficit, and because the synthesis of creatine, the precursor of creatinine, is the main user of methylation in the body, the rate of excretion of creatinine can vary from the norm. I don't think this has totally sunk in yet, either.
Best regards,
Rich