Concession Treatment as a Resident

Qn: Does the first consecutive year start on the first year a non-resident exercises employment in Singapore or the first consecutive year only starts counting after the first year of employment?

Qn2: Base on the quantitative test, a foreigner will be taxed as a resident if the period of stay in the preceding Y/A is more than 183days. Must he meet the 183 days ruling for all the 2nd and 3rd consecutive years of assessment in order to be granted with the concession?

Qn3: Can the foreigner opt not to accept the concession, i.e. opt not to be taxed as a resident for the entire three Y/As since there are some advantages of opting to be a non-resident?

Qn4: If in the first Y/A, the IRAS taxed the foreigner on a non-resident basis as his stay in the first year is less than 183days, and even tax as a non-resident for the rest of other Y/A consecutively (assuming he exercises his employment less than 183days in the preceding Y/A). how does the IRAS re-compute or adjust the basis of computation for that non-resident if concession is granted, i.e. he is regarded as a resident for all the 3 years of assessment?
Andy