Work backwards from the first day fields are workable, and do not apply until the offer letter matches that plan. TN is granted for a period of up to three years, tied to the employment described, so a status obtained against a schedule that later moves creates a mismatch rather than a head start.
Seasonal work rewards precision, not earliness
Sampling seasons open when frost leaves and close when the crop is in, which gives an agricultural employer a narrow window and a strong temptation to get someone across the border early. Two things go wrong when that temptation wins. Applying months ahead against a letter describing duties that begin in April produces awkward questions about what she will be doing in February.
Beginning work remotely from Ontario for the American company, to be useful in the meantime, raises a separate question about when the employment actually started and who is employing her. A Canadian citizen needs no visa stamp and can present the complete package at a port of entry or preclearance, which is quick once the file is ready, so the sensible sequence is to have the documents finished and held, then travel when the start date is real. If the employer files Form I-129 instead, allow for adjudication before the season opens.