What if the US President is decided by a faithless Elector?

[LittleFuzzy]

So...Michigan's votes wouldn't have mattered at all?

Well, they ARE 16 votes. If Pennsylvania hadn't ended up going to Trump than Michigan would put him over the top if/when his current lead is certified as the final count. As it is, no, Michigan's votes are just going to pad Trump's margin of victory, wrt the electoral count.

Then how is it that Electoral College votes are the deciding the winners....when their votes aren't officially registered until December?

Because that's how the system works, the electoral votes are what determines the winner, not the overall public count, as one of the measures set up to prevent the larger states from completely dominating and marginalizing the smaller ones. And because the system was set up in a pre-computer age where everything wasn't connected 24/7 (and because there needs to be time provided for recounts and other measures to ensure the results are accurate in close races) the system was created with a month-long margin of time between the general election and the official vote of the Electoral College. Some similar reasons are part of why there's more than a month between when the Electoral College votes and when the President-Elect is actually sworn in.