If B12 has an effect by changing the rate of genetic expression, then taking B12 seems equally likely to produce a bad effect as a good effect. Is ME due to one or more proteins being overproduced or underproduced? My experience, both tablets and an injection, seems to indicate that B12 level isn't a factor in my ME symptoms.
I just did a quick check into cyano vs methyl (and several other types on the market). The body seems effective at changing the various forms into the form that the body needs. Marketers may play on that, claiming that their form is better absorbed, or that avoiding conversion is far superior, but any difference can be overriden by how much you take, so I don't see it as a factor. Some people may have a rare disorder that reduces conversion efficiency, so they might find one form superior. I probably get adequate amounts of B12 from meat, so any extra from supplements, regardless of conversion efficiency, shouldn't make much difference.
I haven't encountered anything to convince me that I should bother testing different forms of B12 or trying methylation fads. If methylation rates were truly critical, as market hype claims, I think my doctor would test for that and prescribe treatment. Since that isn't happening, I'm assuming that there's not yet enough scientific evidence to support testing and treatment (except for the few people with very specific disorders).