I interviewed and obtained a written statement from Horatio Hottentot in which he admitted to stealing the jewelry.
Comma after 'Hottentot'?

They cut staff at the plant in early January 2002 so as to prevent closure of the business.
Comma before 'so'?

Thank you for your letter of January 24 in which you requested ....
Comma after '24'?

Hi Victo,

Let me ask you where you would pause briefly, if at all, in saying those sentences. Emotion: smile

Unrelated to the comma question, there is no reason to use "so as to." Just say "to."