Will I be barred from traveling to the US in the future, after a tense immigration interrogation?

A way to increase your chances of admission on your next visit is to explicitly and proactively address as many the red (or orange, yellow 😉 ) flags as you can.

The officer’s two main concerns (apart from being a Nazi, plan a terrorist attack or sell illegal drugs) are:

• You may want to stay in the country and not return home, that is, immigrate illegally. The main argument against that are ties to your place of residence at home.
• Even if you actually will return, you may need to or want to work illegally during your stay. The main argument against that is loads of money in your bank that you can access with a credit or debit card.

What you want to avoid is appearing as an outright work migrant or as a hippie, a drifter with neither plan nor money who is likely to strand in the U.S.

The impression that you are instead a legit tourist can be instilled in the officer by presenting corroborating evidence. Examples are listed below. None of them is absolute proof of your intentions — it’s rather that you tell a consistent, plausible story that fits with facts you present.

• Have a return ticket. Print out your E-ticket.
• Have sufficient funds to sustain yourself. If you plan to use credit cards bring a recent account statement showing enough money that you can live off during the vacation.
• As a freelancer, you may have an ongoing project for a customer that continues after you return. Have a copy of the contract with addresses etc. to show that you have a reason to return. As an employee, have your work contract.
• Same with your home. Have a copy of the lease or proof of ownership.
• Any family staying home helps. Provide names, relation and contact data.
• Prepare a detailed, documented narrative of what you are planning to do in the U.S. That could be a little folder with a map, rental contracts for AirBnB, hotel bookings, RVs, rental cars, contact addresses etc. Have a written schedule showing where you plan to go on which day. If you are staying with friends the entire time (hence no accommodation booking), have their address and phone. Be ready to explain how you can stay there for so many weeks. If you actually don’t know exactly what you’re gonna do — a lack of planning which the type of person who is an immigration officer may not understand — invent a few brush strokes that stay as close to the truth as possible. Make sure it’s plausible though with money, time and the rest of the story. National Parks are always a great idea, in fiction as well as in fact ;-).

Now, this is likely overkill: Probably, the officer will only look at a few key items (ticket, money), if at all. But if push comes to shove you are prepared: You’ll tell a good story that makes sense, fits the available facts and that they are happy to hear. You’ll not be insecure and start stuttering or improvising.

Addendum: As it happens, the Guardian has a story today about an Australian young man, Mr. Dunn, who was detained on arrival in the U.S. and eventually sent back. The specific circumstances of his detention are deplorable and somewhat traumatic, but the story supports most of the points I listed (return ticket, funds, ties to home, consistent story corroborated by evidence):

He was interrogated by a US Customs and Border Protection (CBP) officer, who refused him entry after determining that he had not booked onward travel beyond Mexico.

(This is somewhat remarkable as Mexico is already outside the U.S., which should satisfy the officer. But the officer was perhaps concerned that the cheap trip to Mexico was only a pretense.)

Dunn tried booking a flight to Panama, but did not have enough money in his debit card account. […] The officer also questioned Dunn about his inability to book the onward flight out of Mexico, and whether he had enough money to support himself in the US.

The administration concluded:

“You are inadmissible for admission into the United States … because of your inability to overcome the presumption of an intended immigrant. You have no ties or equities to your home country or sufficient funds to support yourself for your intended period of stay,” he was told at the end of the interrogation.

I have a journalist friend who was an editor at a very prominent counterculture publication and reliably, always, 100% of the time got pulled aside for extra screening when crossing borders. He had a name for the face they make when they spot his name on the list, I forget what he called it but it was frequent enough that he gave it a name. But, he always got through. So, based on that, I’d venture to guess that as long as you didn’t do anything worse than have your name regularly published in the masthead of a nationally-known counterculture magazine, you’ll probably be ok, although if you are on a list you may have more secondary screenings to look forward to in the future.

I was pulled into secondary once as a Hungarian citizen and Canadian resident and a few months later I applied for and got my NEXUS card. Not only you do not get barred just because of a secondary, you do not even become ineligible for Trusted Traveler programs. (UK citizens are eligible for Global Entry.)

Admittedly, if I resemble anyone famous, that would be probably be Humpty Dumpty.

Given that you were admitted, you have no reason to think you will be barred in the future. There’s a good chance that you can expect to be pulled into secondary inspection on future visits, however, as you probably have a flag in your record.

Wish I could prove to them that I really did leave with my return flight ticket after my holiday.

If you fly out of the US, the airline will report your departure. If you cross into Canada, the Canadian border service will do the same. If you leave through Mexico, you’ll have to tell them yourself. You can keep track of your record at https://i94.cbp.dhs.gov — there are instructions there for reporting your departure if they don’t pick it up automatically.