From what I've seen, I guess both compounds might be in circulation right now, mexedrone appears to be beta-methoxymephedrone indeed (the structure in the first post).
I've read some weird posts on one Polish forum where some people state that mexedrone (beta-methoxymephedrone) is very weak, almost inactive, however, upon contact with air for a few hours it magically becomes much more potent. Very weird I thought. But then I started wondering if slow condensation with the removal of CH3OH could be possible, the resulting product would be an enone, also unstable under humid conditions due to the presence of amine (basically methylamine with a substituted vinyl group), but perhaps stable enough to be ingested. It would be very similar in structure to mephedrone itself but with an alpha-beta double bond.