One of the vexing problems about this debate is that there's a well-established cumulative effect to Mercury exposure. But the company behind any one vaccine can say, with a mostly straight face, that carefully-controlled studies show that theirs doesn't cause problems. The operative phrase "by itself" is left unspoken. Unfortunately, the trend has been to to give kids more and more shots at an early age. There's also some evidence that the proteins responsible for scrubbing the body of heavy metals are missing in some people, leading to an uneven distribution of cases. A shot of bourbon by itself probably won't cause lasting problems, but a bunch of different whiskies all at once might, particularly if you lack the genes for metabolizing alchohol.
Yes, that is a huge problem. The FDA should be funding research in matters like that, but they have neither the budget or the will. The problem of different drugs and substances causing problems when mixed even though none is dangerous on it's own is well known but not nearly well enough tracked.
Jay