Can “has” replace “have”?

"Has" must replace "have".

Let's change the sentence a little:

Her husband, along with the entire Singaporean police force, has left supportive messages on her post.

The verb agrees with the single subject, not the words within the commas.

Her husband and the entire Singaporean police force have left supportive messages on her post.

Joining the two with "and" requires the use of "have".