Consider getting a statement signed by all parties which states that everyone has agreed to terminate the current lease early on X date and all obligations by all parties have been paid in full and that the deposit will be kept by the landlord and will be applied to the new lease. 

Have about 50k I would invest (access to more), with excellent credit and good income too, from my full time job.

If they fail to pay the new full amount send a pay or quit for the missing amount with late fees.

I started my professional career in medical device sales and still do that full-time.

I hired Jenny Yi to design our latest STR in person, she demanded that I pay the full amount of her services upfront, $7,500, which I thought was strange as we are used to paying half upfront and half once it's completed, but proceeded anyway because a couple of people recommended her.

Obviously in the other end of the spectrum than your developments.  

I thought maybe the fund is full, or maybe Brian simply forgot about it.

Full stop.An investor can buy undervalued RE, do efficient improvements and create massive value. 

You should have received your full payment, so give the cleaners their fee.

Went through extra fast because the developer really had no other good optionsI doubt we'll know the full details for years to come but I'm curious if you have an opinion, or if there's other probable options C,D,E I could be missing as just a public bystander to the situation @Rene Hosman It's very rare that anything gets done quietly behind the scenes.