Answer to questions 1: I think this could be a viable tax strategy by running it as a STR for 1-2 years. I like running it at least to the 2 year mark better. Echoing what @Michael Baum advised, there is potential risk with this strategy. The longer you operate it as a STR, in my opinion, the lower the risk becomes. The STR rules have been on the IRS books for a long time. But, the audit risk could increase with the increased popularity of this strategy in recent years.
Answer to questions 2: You would not be able to deduct expenses until the property is placed into service. Placed into service is defined as when the property is ready and available for use. So expenses paid before that would added to the basis of the property and depreciated. Generally, expenses are fully deductible after that date. You would be able to take bonus depreciation on your 5,7, and 15 year improvements after the property is placed into service. This would include items such as appliances, carpets, furniture, landscaping, fences, etc. I am assuming the 15k Mortgage is just the interest portion of the loan payments. Only interest is deductible.