We are having challenges keeping the calendar straight across multiple rental site platforms.

Not sure, I guess that's why it's important to be able to do this in multiple locations. 

We had a tenant complain multiple times about her toilet clogging.

Or if I have multiple properties with positive cash flow that are not mortgaged. 

He is somewhat quite elderly but surprisingly firm and active; is an 'out of towner' and did mention his previous tenant where a pain so the property has been vacant multiple years for this reason (at least that's what he tells me). 

I have recently come across the concept of using a series LLC to hold multiple assets under an umbrella LLC, while maintaining them separately.

IMO unless you plan on generating income with the lic or maybe are getting access to the MLS to invest in an area that is not getting multiple offers, your time/money can be more wisely invested in marketing or networking or actual investing. 

Yea I thought I was applying for an investor intern position, ended up in a KW meeting with multiple others.

So multiple mortgage inquires onto a consumer's credit will not impact the score (within reason).  

Or should I use the equity on my house to make down-payments and purchase multiple duplexes.