No, you are not a captive audience. You may end the interview at any time. Politely tell the revenue officer, "that's it, I would like to obtain legal representation." Revenue officers are sent to your place of business or home without an invitation for 1 reason: for you to show them the money! A revenue officer must respect your wishes to end the interview to hire a lawyer. However, as you mention, many revenue officers will try to pursuade you to keep talking by threatening a summons. Some will go so far as to try to persuade you not to hire a lawyer despite your stated intentions. Don't fall for it. An IRS summons is a measure reserved for situations where a taxpayer is being openly evasive, but you have a right to a lawyer.
When you ask for a lawyer, you may see a sudden shift in the officer's demeanor. They may tell you that you won't get any better deal with a lawyer, or that they will now be nice to you if won't do it. The value of a lawyer in this situation should be apparent. Knowledge is power, and the only safeguard between the IRS and the taxpayer is PROCEDURE. If you don't know the procedural limits of the officer's power, they darn sure won't tell you.