I disagree with StoneyB on this one. You're describing a collection of events all prefaced by "if". It is split into several sentences, but all of it is based on the conditional.
Had "when" been used instead of "if", it's usage with a past tense description of the condition ("when she happened to be busy"), would make it a narrative about past events. Sometimes she was busy and sometimes she wasn't. On those past occasions when she was, this is what happened.
"If" describes a rule to apply to future events. Combining that with a past tense description of the condition changes the perspective of the narration. It describes events as if they haven't happened yet; taking the reader back in time to before the event and explaining from that vantage point what would happen in case the "if" condition is true. You can't use past tense for any of what is based on the "if" since you're describing things that haven't happened yet (in the timeframe of the story).
So it should be "would occupy" and "would take".