[defen]

Even with the misspellings it's obvious what words you meant. If someone forgets how to write "嚔" are they just missing a few strokes but it's obvious what they actually meant? Or do they have zero clue what it's supposed to look like?

[Chathamization]

From what I've heard people say (and what I've seen), the most common way to handle it is to simply write another character that sounds the same. In other words, the characters can be used as phonetic elements when it's needed. It looks weird (in the same way that spelling words phonetically in English can look word), but it's doable.

That's for situations where they had to write something by hand but didn't have their phone with them to check (otherwise they can just spend a second to look at the character), which isn't a common occurrence.

[addy34]

It depends, but it's not uncommon to completely forget the entire character. If you sort of remember it, then the muscle memory in your hands often helps to finish the character correctly once you start, at least that's what I've found and heard from others.