If I am a US citizen visiting Canada as a tourist, can I walk across the border back into the US at any point along the border (as long as I can do so without trespassing in either country)?
Crossing the border at a place other than a border checkpoint does not make you deportable if you are a U.S. citizen, but it almost always violates U.S. border control and customs laws, which, for example, requires in most cases that you present a negative COVID-19 test taken within the past 24 hours to reenter the United States, and in essentially all cases that you make a customs declaration to an immigration officer (even if it is merely to say that you have nothing to declare).
Even if you weren't COVID-19 positive, and if you had nothing to declare, while it still isn't a major offense, it would be still be some sort of violation of the law.
Often, however, a prosecutor would exercise the discretion not to pursue it on the grounds that it wasn't serious enough to justify the public resources involved in doing so if no real harm was done.
No, you cannot.
Except as otherwise authorized by the Secretary, individuals arriving in the United States other than by vessel, vehicle, or aircraft shall— (1) enter the United States only at a border crossing point designated by the Secretary; and (2) immediately— (A) report the arrival, and (B) present themselves, and all articles accompanying them for inspection; to the customs officer at the customs facility designated for that crossing point.
Any violation incurs a civil penalty of $5000 for the first violation, and $10,000 for subsequent violations. Intentional violation is also a crime, punishable by up to one year imprisonment and a fine of up to $5000.