Sherry, it’s a possible snif lehatir.( The caveat being that I’m a baalabos not a rabbi, and it’s been a while since my Yoreh deah farher.) The casks can’t absorb more sherry than their wood can hold. Once they clean it out, it reduces the amount of sherry in the wood. I’m not sure what the cleaning process is, but it’s clear they don’t use a davar hapogeim so we don’t have that one. Also, is it possible to dissect the wood and see how far the absorption penetrated? Sherry is a staining liquid, and wood would reflect it.
My rav is a “so saguru mipnei ish” type of rav. If the research holds up, he will have no problem telling me not to drink scotch. My relative in the kashrus agency is a similar type of individual. He’s not beholden to any special interests. He’s also a teetotaler and will not be affected by the truth however it comes out.