If an individual has a domestic partner filing a separate tax return, can the individual just report his/her own income to qualify for a premium subsidy if they live with a domestic partner who earns a higher income and their household income combined would be too high to qualify for a premium subsidy?

Household income is the sum of a taxpayer’s modified adjusted gross income (MAGI) plus the aggregate MAGI of all the individuals for whom a taxpayer properly claims a deduction for personal exemption and are required to file a tax return. Hence, if the taxpayer does not claim a deduction for personal exemption for another individual, only the taxpayer’s income will be considered for eligibility for premium assistance.
See example on page 15 in the Eligibility for Individuals and Families participant guide.