1. Your primary residence as clarified in IRS Pub 463(below) is Kennewick..... If you have more than one regular place of business, your tax home is your main place of business. http://www.smallbusinessnotes.com/fedgovernment/irs/463/taxhome.html
2. In Ilwaco, you would have a second residence that is not definable as a tax home since you are in Kennewick 3 weeks of the month. If the situation were reversed, then the residence where you stayed the longest, Ilwaco, would be your primary residence and tax home.
3. Yes, if you traveled to Ilwaco for a week out of each month, you would have the normal travel and transportation expenses connected with your business away from home including hotel expense.
However, deducting or depreciating the cost of a condominium which is your 2nd residence is not provided for under the tax laws anymore than would be the writing off of a vacation home. The mortgage interest expense and property taxes would be Schedule A itemized expenses as you suspected in your question above. Your hotel expenses for one week out of the month might very well exceed the monthly cost of owning a condominium; one would think that minimizing tax deductions by owning rather than renting should be a logical acceptable event, but, to date, such an idea has not beome tax law.
From IRS Faqs: (http://www.irs.gov/faqs/faq-kw140.html ), I offer a quote:
""Is the mortgage interest and property tax on a second residence deductible?
The mortgage interest on a second home which you use as a residence for some portion of the taxable year, is generally deductible if the interest satisfies the same requirements for deductibility as interest on a primary residence. Real estate taxes paid on your primary and second residence are, generally, deductible. Deductible real estate taxes include any state, local, or foreign taxes on real property levied for the general public welfare. Deductible real estate taxes do not include taxes charged for local benefits and improvements that increase the value of the property. For more information, refer to Publication 17, Your Federal Income Tax for Individuals, Chapter 24; Tax Topic 503, Deductible Taxes; and Publication 530, Tax Information for First-Time Home Owners.""
4. You might also wish to consider renting your condominium for 3 weeks out of each month as many owners of vacation homes do; then, you would have a business versus personal allocation. As a rental home, you would have the normal business expenses including depreciation and maintenance expenses, but you would need to separate your personal portion of the condominium(25%) from those expenses.
I do hope this information helps you to decide on the best business decision!!
Contact me again for any additional thoughts you might be pondering.