Ask Microsoft. But 1 character would get you a few hundred (or thousand) matches, so I doubt it's apt to be too useful.It's not instant until you've typed 3 characters. Why 3 and not 1?
As I said before, not supported.remember splitter position?
As I said before, not supported.remember hide TOC?
Alias it. I prefer Overview to be the default. Defaulting to Search will only confuse new users.open to keyword search (when no context specified)? I doubt anyone wants the Overview to be the default.