Re: the "loaded cost", back when I worked for MSFT in the early 2000's, the rule of thumb for management was $250k per person total cost. This is when $125k was considered a pretty good SDE salary. So I'd double your $3b.
Yes exactly. There is much more than just "benefits". Cost of office space, computer equipment, office supplies, electricity, janitorial costs, heating, security, servers, IT overhead, etc. When you take ALL these costs and spread them over the company, the loaded cost of an employee is not "mostly" salary.