Although I think this theory isn’t likely to be true due to the lack of evidence, especially in Canon, it still makes a lot of sense to me and feels very fitting for Palpatine. He’s a master of manipulation, and when someone wants to tempt others, they aren’t going to appear to them as a horrifying demon. They’ll want to seem pure, angelic and benevolent.
I personally think these are all good, viable explanations and believe they can actually be combined to form a greater explanation. I propose that Sidious was already partially deformed due to his use of the dark side and utilised some sort of Sith Force power or alchemy to improve his looks when appearing as Chancellor Palpatine (common side effects of practising the dark side include yellow eyes and pale skin, so this would explain the way Palpatine looked in the Maul miniseries). However I also think that the deflected Force lightning led to even further disfigurement and scarring. Perhaps Palpatine’s mask and real face were even morphed or melted together due to the sheer heat and energy generated by both his lightning and Mace’s lightsaber?
Either way, we know that Palpatine always viewed his scarred and disfigured face as his true “Sidious” form. Going back to the Revenge of the Sith novelisation, upon seeing his new appearance, Sidious states “And so the mask becomes the man. I shall miss the face of Palpatine, I think. But for our purpose the face of Sidious will serve.” To him, Palpatine was dead and his face before his disfigurement represented that politician from Naboo. He embraced this new deformed appearance as a visualisation of his true self – evil, twisted and unnatural. And although this subject remains unanswered for now, one thing we do know about Palpatine is that he has an affinity for Force lightning, even if it’s being deflected directly back at him. Seriously, the guy needs to learn when to stop…