> My office is a 5 minute walk from the courthouse, is it feasible to check into the waiting area on the morning that I serve and then go to my office until summoned?

No, you’ll have to wait in the assembly room.

> Assuming I actually serve as a juror, when would the day likely end? I see that the business hours are listed as 7.30am to 4pm, can I assume I’ll be done by 4pm at the latest?

If you are picked for a jury, most judges will excuse you by around 4:30, but it is up to the judge.