Yes, when there are multiple adult occupants, you would typically combine their gross incomes to determine if they meet the income criteria.

When you leave, a duplex will typically generate more cash flow as a long term rental than a single family home.

You can't go wrong anyway but you want to have some sort of boots on the ground to get you started and understanding the area.

There are more buyers for SF homes, but Multifamily properties typically cash flow better as you mentioned. 

Terms such as interest rates and payment schedules are typically negotiable.Combining creative financing with seller financing can include strategies like:Subject-to and Seller Financing: Taking over the seller's mortgage and financing the equity portion through the seller.Lease Option: Renting with an option to buy while negotiating seller financing for part of the purchase.Wraparound Mortgage: Keeping the seller's existing loan while they finance a new loan that includes the balance.To proceed, consider connecting with experienced investors, understanding local legal implications, and attending networking events to gather insights.

Personal loans are typically shorter term maybe up to 5 years.

In these instances typically the lender will pay it to protect their interests. 

@Jeff SkinnerRecommend you first figure out the property Class you want to invest in, THEN figure out the corresponding location to invest in.Property Class will typically dictate the Class of tenant you get, which greatly IMPACTS rental income stability and property maintenance/damage by tenants.If you apply Class A assumptions to a Class B or C purchase, your expectations won’t be met and it may be a financial disaster.If you buy/renovate a property in Class D area to Class A standards, what quality of tenant will you get?

I find these to appreciate better than your typical LTR here which are usually in C/D class neighborhoods.I tend to look at each Real Estate opportunity as what its highest and best opportunity is. 

of those I like next door best because it is hard to fake and the reviews are easily tied to a reviewer in your community.The costs of ADUs vary significantly for various reasons including by ground up vs conversion, difficulty with lot (sloes, access, etc), finishes, and even the size (bigger typically have lower PSF).Good luck