Is there a way I can "co-sign" with one of my parent's credit cards and be on their credit history in order to boost my credit score? 

So $4,500 of additional costs to have a partnership only to avoid hypothetical risks would exceed the profits from many SFH. 

Reports to credit bureaus.

In any pricing software or even inside of the OTA's themselves you can offer weekly discount on stays 7+ nights and additional savings for 28+ nights. 

A cash-out refinance or home equity line of credit (HELOC) can be powerful tools to keep your momentum going.Focus on Stability:Look for investments in areas with strong job markets, population growth, and consistent rental demand.

Those are some of the unique things we offer in addition to helping each investor build a strategic investment plan & roadmap to achieve your specific goals.

If your interest payment is $8,125Your stocks have to make a dividiend of about $11,000 of which you would prolly pay 15% to 25% in federal / state taxes / NIIT.Assuming you will make a 10%-11% return is not guaranteed.You will also feel good about having a paid off property(relief).Furthermore, your credit score may go up with a paid off property.Best of luck!

As for valuation, I see no reasons why a Junior ADU would negatively impact additional housing unit.

Check with your local credit union

Otherwise, you may need to replace a water service (13-15k), electrical service (10-15k) and upgrade wiring (This is a tough one, but maybe an additional 8-15k inside).