The $350 and $700 exemptions are for residents.
I will agree that it appears that the law and policy are extremely confusing.
Actually it is very simple.
You need to think of it this way. The duty allowance for visitors is zero. Zero as in everything is subject to duty. A visitor has no exemption from duty except those items specifically listed in the law. Everything is taxable except those items specifically exempt under the law.
You are allowed to bring in a certain amount of tobacco and alcohol duty free as exempted under the law.
The undefined policy is you are allowed to bring in a reasonable amount of personal items that you can reasonably consume during your visit. That decision is solely at the discretion of the particular Customs Officer you encounter. Sunscreen, food, personal items like toothpaste, deodorant and shampoo are examples of personal items.
Anything a visitor brings to leave (in a condo you own or otherwise, gifts included) is subject to duty. No exceptions under the law.
My understanding is that you generally do not get questioned over small dollar amounts. I don't think they want to hassle with small dollar amounts of duty. (As as I say that, I assume I will get grilled over a dollar on my next visit.)
If you can relate it to the income tax in the US. In one simple section, the Internal Revenue Code states that income from whatever source is taxable. The rest of the code defines exceptions to that rule and deductions from income.
Finally, take the example of sales tax. All sales of tangible products are subject to sales tax unless otherwise exempted.
Don't think of things in the other direction, meaning nothing is taxable unless specified as taxable. That type thinking is backwards and wrong.
Sorry to take up some much of your time on such a long answer, but I hope it helps you understand the theory behind the imposition of the duty.