I have a cosigner willing to help, but Im unsure whether they must be in my LLC or can just be a guarantor, has anyone dealt with this before?

R.E. agents/brokers now have ALL tools, options, networks readily available. 

It's not simple, sorry.If you have not closed, consider tax mitigation strategies such as a 1031 exchange.

All residents but the smokers have lost use of their balconies and keep windows closed now.

Have them send you a line by line item of what's needed.

To thrive in real estate investing, you must maintain a firm grip on your finances.

but keep in mind CA has higher taxes and reg fees and I’m not doing more than 1-2 flips (short timeframe flips)  a year and many years do 0 flipsDo most flippers put flips into LLC Or in personal name (sole proprietor)  in CA (where you must pay $800 yearly plus accounting and other fees $300to $600+ yearly to maintain LLC) Pros for personal name-save on taxes accounting.

How many properties have you bought?  

I'm surprised someone verbally told you you can convert back to a duplex and didn't tell you that you must get permits.from what you are describing it seems the town gave you a hard time because you didn't get permits, I don't know that you have any legal standing to go after the town,  again, I am not an attorney 

What are the best areas of a single family house to renovate that will have the greatest improvement to home value?