> Autocompletion to the nearest ambiguous prefix. If I type “g” and that matches both “Google” and “GoodReads”, this operation would fill in the two “o”s, allowing me to then type either “g” or “d” to select the option I want

This is terrible, unless I misunderstood. It would mean that if I typed "Google" without looking at autocompletions, I would end up with "Goooogle".

I keep fighting the in-browser auto-completion boxes which try to be too smart and annoy me to no end. Whatever auto-completion you provide, your first priority should be "do no harm": if the user typed something that should be considered as Holy Scripture.