It is not a indication of when rent is officially late.

Has anyone ever obtained a secured loan from a bank using a pool of Louisiana Tax Liens (Tax Sale Certificates) as collateral?

After a couple years of testing the life of a landlord, I decided to officially go for it around this time last year!

However, are there any issues with zoning when increasing the # of units from 4 to 5 since 5 is officially commercial?

Aside from having to put 10-20% down to secure the loan, what can I expect to encounter when dealing with hard money lenders in terms of interest rates, points, getting the funds from them to pay contractors/buy supplies, etc.

While anyone with the proper securities licensing can sell them, your typical financial advisor doesn't know real estate. 

He adds they haven't really lived there in two months for more than a couple days because they feel sick there.- Today we offer to terminate the lease for the remaining 3 tenants but we hold remainder of the security deposit.

They have responsibilities (e.g. wife/husband, kids, mortgage payments) and taking that dive into action represents putting their security at risk.

Is there a way to legally require a larger security deposit?

I would do 2nd mortgage with the interest as high as allowed by your state to secure your investment and the balance of the promised return should be in your JV agreement in the LLC.