I have little to doubt that Singapore will side with the US. Its military is too close to the US, and a large chunk of its younger population are very anti-China. Singapore will definitely suffer for this, as China is responsible for the largest chunk of trade going through the Malacca Straits. But in this day and age, it is better to not underestimate stupidity. The Germans have proven it themselves. Some Singaporeans might think that the non-China trade could compensate. Some fools might even say that the suffering is "worth it". Nevertheless, I'll be happier if Singapore does prove me wrong.
I expect ASEAN to be indecisive on what to do if the Malacca Straits is blockaded by the US. Some nations like Singapore and the Philippines might foolishly support that. Some in ASEAN would be against it. Others would be indecisive. The most important ASEAN member in such a scenario is Indonesia. Can Indonesia offer an alternative route for China through the Sunda or Lombok Straits, or will it be coerced too?
A Chinese naval drill near Singapore would send the message who the big boss is.