How would I go to clean/reset the log file on a Switch that did "bad things" like NSP installation, CFW/homebrew usage and so on, when it never was online so it should be good to go online then?
If you're looking to clear your logs in order to clean your NAND to go online, it won't work. You WILL get banned. That's why you were meant to make a pre-modded NAND backup.
No, it's already mentioned that there are ways (at least some long ChoiDujour tutorial which was meant for downgrading originally but I can't find that any longer).
Once you hack your switch you leave traces of such activity in it. As of yet there's no means of clearing said data from your console, hence resulting in a possible ban thereof, if the console connects to the internet.
Your only solution would be to restore a NAND backup made prior to hacking and tampering with the console.Good to see that you haven't yet connected your console online and I would advise you not to do so without taking some safety precautions namely using Incognito or 90DNS (though I'd highly suggest you to use Incognito)
In case you wish to play online you can set up idn_mitm LAN play thereafter.