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. 

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

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

Essentially, you set up the LLC and the trust is the sole member of the LLC.

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. 

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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.

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. 

Also, for anyone based in Philly, I've heard from one contractor that quotes of 20-30k were common a few years ago, but due to recent rule changes, contractors need to maintain excavation licenses/ insurances to do any dig out work and that cost is getting passed to customers.