I am about to pay the land cash 500K, then build 10 apartments for roughly 1.5M.

You could also get a line of credit for about $150k (you should be able to borrow 75% of the value.)The reason I suggest this is…1) you save $40-$60k in selling costs. 2) you have a lower blended interest rate (2/3rds at 3.25% and 1/3rd at 7 or 8% instead of the whole $500k at 7 or 8%) saving you another $1k/mo in interest. 3) you only pay interest on that $150k when you actually use it, not from day 1  Unless you hate this property, or want to buy something you can’t afford without selling, that would be my plan.

If your contract states that "time is of the essence", you could have forfeited your right to break the contract and get your EMD back simply by not adhering to the dates specified in the contract.

They're paying for results.  

if the second buyer is paying cash then your done they pay your off and your first note.. off to the next one..

I’m thinks just pay him his regular price for the job and throw him 5-10% finders/gc fee instead of making a 70/30 split or something like that.

Also, it often helps to connect with a local agent/do extensive due diligence on renovated/new construction properties to see what the property may sell for or appraise for (if doing BRRRR method) as that can impact the price you should pay.

We all know they won't ever pay.

You're going to end up paying a lot more interest on a heloc than a 2nd mortgage, if you intend on holding that debt for more than a couple years. 

It may seem easy to jack up rent but there is something to be said about good, on-time paying tenants.As an accountant, I always rely on the numbers.