To keep things somewhat brief, my partner and I plan to eventually start a fund as a REPE, after a few years of building out a diversified “core” HFI portfolio, but don’t plan on investing other people’s capital until we’ve had some experience managing our own mixed asset portfolio for a few years.

I am trying to locate the owners of a few properties I've sent yellow letters to. 

Or maybe try to "psyche" the tenants out into leaving by getting them served again and sending a tersely-worded letter from a lawyer that makes them think that a court hearing/eviction is imminent.

You could find houses that look in disrepair and offer to write letters for wholesalers or Realtors.

You could require Proof of Income - like a pay stub AND that order award letter. 

Second invoice had a description of Plumbing-Install New Shower Mixing Valve (Materials and labor included) charge $350.00.While I understand when things go out and need to be replaced etc. but this seems excessive for the same bathroom, Shouldn't they have had just one plumber do everything. 

For my particular situation I have the funds to provide the deposit in Cash and had a pre-approval letter from a private lender for this property (which is dependent on appraisal, title, and final underwriting decision), but am unsure if they will accept this as documentation or if I will be out my earnest deposit if I go for this opportunity.

Probably your best avenue of success if leads you generate yourself from post cards, calls, letters, and driving for dollars.   

I actually explain this to my tenants three different times at the beginning of a tenancy...verbally, in their welcome letter, and it's also written into the lease that they sign. 

I'm interested in making the owners stakeholders in a new mix-use redevelopment with retail, office, and mixed-income in lieu of purchasing their properties.