Do you have to be American to apply at the border? Planning to have my partner from France fly to the States and apply at the border. But all immigration lawyer websites refer to Americans doing this.
There is a potential, should the TRP be refused by the Canadians, that the person will then have to turn about and be screened again by the Americans for re-entry into the USA.
So immediately upon being refused entry into Canada the Americans will then want to know more about the person and what their purpose for being in the USA actually is - to get into Canada won't be a satisfying response.
Citizens of the USA in the same situation aren't caught in "no-man's land," unable to enter Canada or return to the USA, and so this could equal detention by the USA for any other foreign national because there's no other option.
You could look at Saint Pierre and Miquelon for a contiguous Canada/France border, but really there is a risk to not applying through the Canadian Consulate for the TRP because this could also lead to a refusal and what happens if neither the USA or Canada will readmit the person?
I don't know if you can get out of Saint Pierre and Miquelon without going through either the US or Canada so it could also be a literal dead end. People can try to beat the system but then again, without knowing all of the possible outcomes, the system offers significant reasons to simply follow more predictable paths like applying from where you live.
Who wants to be locked up in an ICE detention center?