What or who do you suggest I speak to or learn from to better understand my options for mortgages?

Keep in mind that your mortgage terms and insurance premiums will probably be less favorable in an LLC.

Most people have a mortgage on their home when they sell it.

New unit: 4-unit multi family in which 3 units (with existing tenants that will carry over from seller) will remain investment; I will move into (currently empty) 4th unit AND USE as primary residence with primary mortgage.

It does seem to make some sense to keep living there with the help of a roommate paying the mortgage.

@Scott GorhamSome of the strategies for buying a property if the original mortgage stays in the seller's name would be subject-to or a wraparound mortgage.

Received access to mortgage portal to ensure all payments are made (including escrow).Anything else here I should do or not do if acquiring for the long-term?

You technically do not have to replace the mortgage. 

Along with that you purchased at a decent price, cheap rates so your mortgage should not be expensive.

Given your mortgage situation I would opt for taking out a HELOC.