Most people either have nowhere to go or think you are bluffing so odds are you will end up in court.

BLUF: Ask a tax attorney. 

And when I asked the other two experienced buyers whether they knew about it, they looked at me like I was bluffing them.

He probably bluffing to buy out more time. 

You could just post a note saying that they muct contact you within 5 days before you enter and remove their belongings as you believe the property has been abandoned.I'm not suggesting you actually do that but sometimes bluffing a tenant is very useful and far cheaper than the alternative.

So other than bluffing and threatening to leave (even though i really can't),   what other leverage do i have?    

He could be just bluffing....At any rate, since you screwed up and gave him 50%, you have few options.

BLUF:  I have real estate, some investments and assets, and some inherited money in a retirement account. 

If it is still a deal, and you feel they aren't bluffing, take it.Just keep in mind at this stage, if you come back and say "give me $10,000 back", the seller could cancel the contract and you are stuck.

I have hit a few bumps on the road since my first property was purchased late November 2017, and am curious to hear some different opinions on what I could do next.I have about ~$65k in available cash to invest (raised from private investors, BLUF I'd like to get it invested ASAP as its not doing any good sitting in a checkings account), as well as good relationships with a conventional and private money lender.