On a 16-unit you'll realize some efficiencies of scale, so could knock that down a couple of percentage points.

It sounds like the percentage they are charging is a regular markup... after all, they sourced the vendors (likely have already worked with them for years, in fact, and so the vendor is more trustworthy and fair with them), coordinated the repair between vendor and property/tenant, and took care of the payments. 

But can the down payments be a smaller percentage?

What percentage of the year are you doing STR versus LTR?

Prices have gotten unrealistic for real estate, so I am prepared for a price correction, or for a long-lasting crisis, depending on the country.çIn Europe the only way to maintain the welfare state is charging more taxes and one of the easiest way for governments to get more money through taxes is focusing on real estate (property tax, a tax percentage on properties that are vacant etc)

In reality rather than a percentage it should be a dollar amount. 

The note rate , maturity time and lates fees etc also add or subtract the discount value (percentage )Best of luck in all your endeavors.  

I may rework the percentages so that the return for 15 months is slightly higher than 9 month return.

Can we get a percentage of the deferred rent back? 

If you don't know how much to subtract from the purchase price for land value, usually what I typically do is use the tax assessment or some sort of appraisal (around me a lot of towns use Vision Appraisal) and I take the percentage breakout between land/building on the assessment or appraisal and apply that percentage to the purchase price.