I found some more definitive answers that I don't think my tax preparer can refute. Still wondering if I will be audited for claiming on the closing attorney's paperwork sent to the IRS that I used [a portion of?] the property for business or rental purposes....
Here's the irrefutable evidence, however, for future passers-by....
See: https://www.law.cornell.edu/cfr/text/26/1.121-1
CFR § 1.121-1 Exclusion of gain from sale or exchange of a principal residence.
(c) Ownership and use requirements -
(4) Examples. The provisions of this paragraph (c) are illustrated by the following examples. The examples assume that § 1.121-3 (relating to the reduced maximum exclusion) does not apply to the sale of the property. The examples are as follows:
Example 1.
Taxpayer A has owned and used his house as his principal residence since 1986. On January 31, 1998, A moves to another state. A rents his house to tenants from that date until April 18, 2000, when he sells it. A is eligible for the section 121 exclusion because he has owned and used the house as his principal residence for at least 2 of the 5 years preceding the sale.
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AND FURTHER EVIDENCE THAT THE PHRASE "ANY PORTION" used in Publication 523 literally means a multi-use property that is divided, allocated...
§ 1.121-1 Exclusion of gain from sale or exchange of a principal residence.
(e) Property used in PART as a principal residence -
(1) Allocation required. Section 121 will not apply to the gain allocable to any portion (separate from the dwelling unit) of property sold or exchanged with respect to which a taxpayer does not satisfy the use requirement. Thus, if a portion of the property was used for residential purposes and a portion of the property (separate from the dwelling unit) was used for non-residential purposes, only the gain allocable to the residential portion is excludable under section 121. No allocation is required if both the residential and non-residential portions of the property are within the same dwelling unit. However, section 121 does not apply to the gain allocable to the residential portion of the property to the extent provided by paragraph (d) of this section
(4) Examples. The provisions of this paragraph (e) are illustrated by the following examples:
Example 2 Non-residential use of property not within the dwelling unit and rental of the entire property.
(i) In 1998 Taxpayer B buys a property that includes a house, a barn, and 2 acres. B uses the house and 2 acres as her principal residence and the barn for an antiques business. In 2002, B moves out of the house and rents it to tenants. B sells the property in 2004, realizing a gain of $21,000. Between 1998 and 2004 B claims depreciation deductions of $4,800 attributable to the antiques business. Between 2002 and 2004 B claims depreciation deductions of $3,000 attributable to the house. B has no other section 1231 or capital gains or losses for 2004.
(ii) Because the portion of the property used in the antiques business is separate from the dwelling unit, the allocation rules under this paragraph (e) apply. B must allocate basis and amount realized between the portion of the property that she used as her principal residence and the portion of the property that she used for non-residential purposes. B determines that $4,000 of the gain is allocable to the non-residential portion of the property and that $17,000 of the gain is allocable to the portion of the property that she used as her principal residence.
(iii) B must recognize the $4,000 of gain allocable to the non-residential portion of the property (all of which is unrecaptured section 1250 gain within the meaning of section 1(h)). In addition, the section 121 exclusion does not apply to the gain allocable to the residential portion of the property to the extent of the depreciation adjustments attributable to the residential portion of the property for periods after May 6, 1997 ($3,000). Therefore, B may exclude $14,000 of the gain from the sale of the property.