The purchase price was 395k and furnishings cost about 20k.

They cover too large an area and will lead to you either getting take advantage of or making a costly mistake:(Investing in the City of Deroit should actually be done block-by-block.

My preference is to actually have to separated as cost efficiently as possible, but also wouldnt mind the ADU and main house shared meters as this would save me costs on my initial construction. 

Those losses can offset W-2 income when using cost segregation.

I believe alot of owners in the area are under insured especially when it comes to loss of income and rebuild costs. 

If you are going to JV for land purchase I would definitely get quotes from architect, engineers, and estimated civil cost.

Otherwise, renovate the garage into a man cave, while keeping the garage elements to easily convert back with no or minimal cost in the future.

Investing with foreknowledge that property values are falling seems like a timing problem or at least a situation where my offer would be reduced to reflect this coming change.As to how much to put in as a down payment that depends more on your numbers, your risk tolerance, ability to get financing, cost of financing, etc. 

We have struggled for years deciding whether to sell or hold because the tax implication to sell is very costly.

This is tough.Depends on how fuzzy you want the business/family line to be.The only thing I'd be absolutely sure of is that your money has more tangible security than being a "personal loan" EVEN IF you cut him a break on cost.