If I am unemployed for a long period of time, can I withdraw my accrued benefits to meet living costs?

No. Your MPF benefits can only be withdrawn under the following situations:

Attaining the retirement age of 65 years
Total incapacity
Terminal illness
Death
Permanent departure from Hong Kong
Meeting small balance provision (essentially less than HK$5,000)
Attaining 60 years of age and permanently ceasing the employment or self-employment