I don't think there's any scenario in a war where Malacca isn't well defended by China. It borders the Asian main landmass and Indonesia. Indonesia and Malaysia have fuck all air defenses and they're likely to be friendly to China anyways. You'd have around the clock ISR coverage and missiles used to snipe at suspicious ships, ASW over flight etc. Maybe some destroyer flottila physical presence.
Malacca is also crucial to prevent EU attacks on China. If they want to get to Asia in a timely manner, they must go through Malacca or Sunda. Otherwise, they have to go to US and then circumnavigate.
Only if India joins, the Indian ocean side of Malacca will be at risk of surface based fires and could be too hot to use. But in that case, China would rely on an Indus plateau plan using missiles fired down at India from Tibet instead, which would distract India once hits on infrastructure start ramping up.
