If I was assigned to Hong Kong in the mid of the Hong Kong tax year, will I be subject to Hong Kong Salaries Tax on income before my move?
No, you will only be subject to Hong Kong tax on income sourced to Hong Kong, i.e. income derived from Hong Kong on account of services rendered here.
Will the IRD consider whether it will be beneficial after electing for joint assessment?
Yes, the IRD will compare the result with and without joint assessment and will only assess you and your spouse on a joint basis if it is tax beneficial .