If you rent it more than that, let's say for 15 days/year, do you suddenly become eligible for 15 taxable days, or just the days extending beyond 14, in this case 1?

Something more like "If my calculated taxable income was 160K, what tax bracket would I be in".

The new standard deduction is $12,000, so that leaves them with $5500 per year of taxable income. 

If your marginal bracket is still 15% after all your capital gain is included in your taxable income, then your depreciation recapture rate will also be 15%.Just how I see it.

Easy to understand and well written.You cannot deduct your down payment or inspection costs from your taxable income, regardless of the dates.

While I'm aware of this, never looked that much into the technicalities of the issue. the risk is if the option cancels the option money the seller receives is now taxable.. so seller needs good council on this as well 

However, that means more taxable income come tax season which is a whole other discussion/consideration.Situation #2: I have a primary SFR that I purchased with 20% down and have been making an additional principal payment every month for the last year.

I know there are benefits to having your AGI low enough so that you can apply 25% or so in passive losses to your taxable income...not sure what the thresholds are for that though. 

How did a cash purchase on this vacant land effect your taxable income?

Your actual taxable income and deductible business expenses should be the same.