Is Vantage Homes coercing the seller to breach our contract???

[Account Closed]

I've been in contract since January to purchase a property that is contracted to close by 5/31.  I have funded escrow and completed by side of the agreement.  

Because there were some boundary questions that I wanted resolved before closing, we wrote an amendment in January stating that: 

The seller didn't resolve all the boundary issues (the neighbor's garage encroaches) and the deadline to close is fast approaching.  I funded the escrow account and let the seller know that it's OK with me to close despite not resolving the encroachment, that I would resolve it myself after closing.  

The seller has been taking his sweet time to sign the closing documents and I just found out why:  Blaine and Jesse of Vantage Homes submitted an offer to the seller back in April that was $20k higher than our contracted price.  Then just today, I found out that Blaine and Jesse have increased their offer to $30k above our contracted price.  

The seller is saying that, since he didn't resolve the boundary issue, he can just wait out the contract until 5/31 and then sell the house to Vantage Homes for higher than our contracted price!  

I'm about to claw my eyes out.  How is this possible??  I spoke with a lawyer who said that, if there wasn't a specific consequence to cure the boundary issue, like extending the closing deadline, then the seller may have a case.  How can this be??  The amendment was written for my protection, in case I didn't like where the results of any boundary change ended up, so I could back out if needed.  NOT so that the seller could back out because he got a higher offer!  

This is just killing me because this deal was going to be the one that was going to finally set me on a path to financial freedom.  I was going to live there and develop this property (it's industrial zoned) and then cashflow it long term.  I tried to contact Vantage but just got voicemail.  I wonder if they'd be as interested in the property if there's a lis pendens on the title, or whether they're doing anything that is bordering on illegal by trying to coerce the seller to not honor the contract?  I just want to talk with the Vantage people to straighten this all out.  

Does anyone have any suggestions about what I can do??  This is crazytown!