TD is framed around a spouse and unmarried children under 21. Cohabitation, however long, is not automatically treated as marriage for this purpose, and provincial common-law recognition does not decide it. A partner in that position should get individual advice on routes that do not depend on the TN holder.
Ask about the partner's own basis, not a workaround
The productive question is not how to make a cohabiting relationship resemble a marriage, but what the partner independently qualifies for. That depends on their citizenship, their own profession and credentials, and whether study or another category fits. Counsel can review both cases together while keeping them procedurally separate, which also avoids inconsistencies between what each person presents.
The position must be prearranged professional work in a listed USMCA occupation, and the applicant must meet that occupation's specific qualification and citizenship rule. Decide early which citizenship the application relies on, and make sure the employer letter, any filing and the passport presented all reflect that choice. Do not invent a universal permitted onboarding percentage or use a professional title to hide a different substantive position. Some accept a baccalaureate or licenciatura degree; some accept a state or provincial licence; a small number accept post-secondary diplomas combined with years of experience.