Can Hong Kong Entry Health Declaration Form be filled in on the spot?

Of course. The entry health declaration form for China and Hong Kong only requires pre-boarding inspection, so it is ok to fill it out before boarding, and it is also ok to fill it out on the spot. China and Hong Kong generally refer to China Hong Kong Special Administrative Region. China Hong Kong Special Administrative Region (hereinafter referred to as "Hong Kong") is the full name of People's Republic of China (PRC) Special Administrative Region.