@Applewood, without reading all of the comments on this thread, I will attempt to answer your questions as to why we are questioning the legitimacy of Mueller's investigation:
1. Mueller has hired a rogue's gallery of highly partisan and openly anti-Trump people. There can be no expectation of objectivity from this group.
2. Regarding the charge of "fixing the election", there is simply no evidence that there were any real attempts made to fix the election, and if the Russians had been successful in such an attempt, they would have most likely come down on the side of Clinton. If there were any credible attempts to fix anything, I would agree with you that it should be thoroughly investigated and chips fall where they may. There's just nothing there.
I do have some questions about Trump's business dealings over time, but even more about Clinton's. However, none of that is being investigated.