Apply for a dimensional variance on the vacant lot to allow a smaller lot and/or setback so it can be built upon.Your best first steps are to sketch a plan, possibly with an engineer's help, then contact the zoning department to discuss your idea with them to get informal feedback on your liklihood of success before you spend a lot of money.

Due to the confusion on their end and their slow response, it's costing me more money than if I just went directly with the vendor from the start.

Examine the bottom half from every angle - how much you can make, what else you can get for your money (don't worry about how much you make on these, they are time suck and the lower half of your portfolio)That's a good place to start.

Did you guys ever raise private money?

Quote from @Chris Garnes: Sounds like you need the money for rehab? 

Maybe, but so far the reports are about neighborhoods where people have real money.

Always have rent money coming in from the duplex vs SFH (not).2.  

Explore my personal favorites, Set For Life by Scott Trench or The Total Money Makeover by Dave Ramsey, for invaluable financial insights.3.

We all invest in real estate in order to make money, and keep in mind that someone who can only afford to live in a mobile home and seller-finance to buy the land is probably tight on funds. 

You're either doing it yourself, spending a little money and hiring/managing someone else to do it, or you don't do it and let the leads rent from someone else.