Interesting analysis but I think the last sentence sums up my sentiments: "Even if all the other issues could be dealt with and mindfulness meditation did improve arithmetic, what is it a valid proxy for?"
My Zen teacher talked about some Shamatha Project study that found sustained concentration meditation to decrease neuroticism and increase conscientiousness. Anecdotally, he confirmed that this is what he has seen as a meditation teacher.
I find it really weird to measure the efficacy of meditation by arithmetic puzzles, though. Is anyone claiming meditation makes you better at math? That doesn't seem like a big area of interest to me.