If you are unsure of how to use the tool for resetting formatting, please refer to this thread, which already goes over this, and has a link to the FAQ page on how to do it.
On neither the Mac nor PC is there a need to plod through one by one to change your font settings. Most of the commands in the Documents menu are bulk commands. If you have any further questions on this, it’s best to post them there. No sense in having two different conversations about the exact same thing.
These are styled though, right? That’s no problem if so, this command is smart enough to not only only reformat text with no style, but it will also update any out of date styled text to match the central designed look.
If it isn’t styled, then it really should be. Raw formatting like that is, in my opinion, of no use save for setting up styles. I recently wrote a how-to on getting your text marked correctly for its purpose. Note these instructions are for Windows, which has weaknesses where it comes to editing Scrivenings text. If you’re using a Mac you could probably convert all of your email formatted text to a style in one single move, provided they are all formatted the same.
Finally, as to the feature request: it is unlikely that something like that will be provided in Scrivener, given that it works somewhat at odds with its primary mode of usage, and what people expect of it. There isn’t a way to dynamically display the text using formatting it doesn’t have, in other words, meaning it would have to be physically changed—using processes similar to the above. It’s one thing to tell someone how to use the tools we provide, as a deliberate decision they make and oversee, and it’s quite another to just do that without any request to do it, merely because you used a PC that day. As you said yourself, that would be destructive in cases where one doesn’t have a properly styled document.