I bet they wait until January or February so they don’t have to pay the tax on those gains for another year (which is why we may be seeing less selling this month than in a normal cycle).

Any advice on what setup is easiest and most tax advantaged?

If you live there 2 years or more before selling you will be exempt from tax on the 1st $250k in profit which is a nice bonus. 

Are they going to be able to buy your proudly done flip.3) If you misjudge the market, do you know when to dump it.4) Can you put a spreadsheet together on your purchase costs, carrying costs, rehab costs, closing costs, selling costs, tax costs, contractors that don't show and delay your project etc. etc. etc.5) Is the return worth the risk.

Especially if your son could then try to sell his primwry with the ADU under the sec 121 tax exemption I wonder if he could sell you the land?

There are tax benefits to running everything through an LLC, but again, seek advice from a tax professional.

Yes, do the tax exchange if you can.  

I would search the BP forums because your tax / legal related questions likely have already been asked.However, it doesn't hurt to still ask the forum what your questions are.

I would be beneficial to determine if they are other aspects of the property that make it a worthwhile investment (tax, appreciation etc). 

Did you inherit this home with a step up in tax basis or will you need to pay taxes on any gains if you sell?