I would assume that disabling Lockdown Mode means wiping the phone to factory condition. Otherwise Lockdown Mode is only as secure as whatever PIN or password you use to disable it, which isn't particularly secure at all.
Yes, but if an attacker has physical access and unlimited time, you've probably lost anyway.
What this seems to be focused on are the "remote zero-click/one-click" vulnerabilities we've seen, in which either a message is delivered that never shows up but installs a backdoor hook, or a website can deliver a malware package to a particular user and install the backdoor hook without notifications.
It sounds like it does improve some of the physical security features, which should help reduce attack surface, but I wouldn't trust any bit of consumer electronics against a sustained physical attack by a sufficiently motivated adversary.
Sounds to me like it's targeting all the zero and one click exploits we've heard about over the last few years. Not having SMS/iMessage download and "parse" random files/formats and tightening up Javascript attack surface to not include JIT optimisations would probably have helped Jamal Kashoggi and his friends/contacts.
Even with this, there's not very much you can do against a state level actor who had physical control of your device and you, and a $5 wrench. Even without having you and being prepared to use violence, a sufficiently motivated state actor will probably get into your device anyway - Apple didn't6 cave to a judge when the FBI wanted them to break every iPhone user's security to get into the San Bernadino shooter's phone, but they didn't get to set a precedent there because someone else broke into that phone for the FBI anyway and they dropped that case...
