How easy is it to arrange a Housing Benefit tenant?

Tenancy Types and Management

I have a potential tenant who is currently renting a 2 bed property and wishes to move into 3 beds. Good character references from landlord and employer. They say they are entitled to a higher Housing Benefit payment, based on the Government's calculator: https://www.gov.uk/benefits-calculators

My questions are:

1. What guarantees are there that the applicant will be given the higher allowance if all circumstances by the property remain the same?
2. How long might it take for the higher allowance to be granted (so that there are no rent arrears for me), especially now in COVID-19 times?
3. Am I better off rejecting the tenant and looking for someone more financially reliable?