Here's a bit of clarity for the two questions related to business use.
Percentage of time spent conducting business in the office - This must be 100%. You may only be in that office for one hour a week. But when occupied, it's for business purposes and absolutely nothing else. That's 100% business use.
Percentage of business conducted in that office - Could be less than 100%. For example, if you are a roofer you spend about 70% of your time out of the office nailing shingles. But you spend 30% of your business hours in that office doing things like payroll, ordering supplies, taking calls from customers, etc.
