I can say in Detroit the wholesaler your talking to has most likely found that deal from 2-3 other wholesalers, they literally have to payout 2 - 3 other parties to get the deal and this is what the fees are high and the deal makes no sense.

As long as we would be legally allowed to proceed with this plan, then the question remains, as you point out, what is the tax burden for the partial 1031 exchange.

It is legal and very lucrative in some parts of the US. 

Once the docs are signed you cant just change the terms without both parties agreeing..

It seems duplexes are valued on rent rates for the most part. I

Is there any legal action one can take or is it just a lost cause?

When you’re confident in your numbers, you can negotiate with clarity and avoid being swayed by emotions.Understand the Other Party’s MotivationEvery seller has a reason for selling.

Whatever is closest to "Legal". 

Usually the hardest part is coming up with the down payment.

I'm not a CPA, no legal advice given.