Given a travel related situation, and given a specific question about the situation, summarize the question into a title

Q: I am a US citizen who will have worked for 89 days in a Schengen country while on a 90-day tourist visa.

What exactly is my best strategy at US border control? Should I admit to working while travelling, or lie and say I was on vacation?

I fully intend to declare the income earned on my taxes for this year, but I simply want to minimize trouble at the border.

A: Returning to US after illegally working in the Schengen area
****
Q: I travelled alone in Japan for business, for 10 days. I declared that I bought items for $550, well under the $800 limit. After my non-stop flight from Japan landed at YYZ, I was directed to secondary inspection at approx. 5 PM. The female CBSA (Canada Border Services Agency) officer inspected all my belongings and the receipts for my purchases.

At 5:15, she started alleging that some other items (like clothing, 1 backup pair of eyeglasses, 1 pair of sunglasses) were bought in Japan, as they looked too new and clean to have been bought in Canada. Of course they did...I'm a business professional! But they aren't that new in fact, and I bought them in Canada.

I repeated that they were bought in Canada and were brought by me to Japan. She said that she didn't believe me. She demanded proof that they were bought in Canada. I answered that I bought the clothing in Toronto and can't remember exactly where or when. I told her the name and approximate locations of some opticians that I remember. It feels unreasonable to expect weary travelers to remember details on the spot, without records on hand.

She then got angry and rude. She said that I was being evasive and warned me not to lie. She threatened to seize my items. She admonished that I, not she, had the onus of proving to her that I bought these items in Canada.

At 5:45, I requested politely to speak to her supervisor. She asked why. I answered that I was a little jaded from being reasked the same questions and repeating the same answers for 30 minutes. She got more offended and ruder, when she raised her voice to ask why I thought she was wasting my time and asking irrelevant questions. I answered that I never said this at all. She said that I don't need a supervisor, if I had nothing to hide.

At 6, I reiterated my request for a supervisor, and she bitterly told me to be patient. At 6:20, a supervisor showed up, and we repeated the same questions before I was allowed to leave at 6:45.

In all, I spent 105 minutes in secondary inspection. My tone was always deferential and polite.

What can be done in the future, to avoid this kind of wrangle, and waste of time? What could I have done better?

I don't intend to complain against her...I'm too frightened of retaliation and being black-listed.
Has this wrangle has already put me on some list and blemished my CBSA records?

A: How do I stop a CBSA officer from wasting my time, by falsely alleging that some of my items weren't bought in Canada?
****
Q: I'm aware there's a form you can sign if ground staff at an airport doesn't want to let you to board an aircraft due to ignorance regarding the destination's entry (or in this case transit) requirements. This way, the carrier's liability is waived if you don't get into your destination.

What is the form called? In other words, what exactly should I ask for?

More likely than not, I'm going to have to ask for this at Tbilisi airport, from where I'm taking a flight to Doha in 3 days (in transit to London). As I only hold an ID card, which is accepted for the UK (my destination), I don't expect to get an easy time boarding the Tbilisi-Doha flight (even though I'll not be entering Qatar), because Timatic doesn't explicitly state that I can transit Qatar on my ID (nor does it state the opposite). Hence, such a form would be of great help.

I've read about this in this blog, where the author, who had trouble boarding a flight to Minsk for transit with her Portuguese ID card, writes:

I wrote some haiku style poems on my phone until the airline man came with a sheet for me to sign: 'It's to make you responsible for coming back, in case they don't allow you to stay in Minsk or enter Georgia.'

A:
What form waives the airline's liability in case of an entry refusal?
****