I'm looking to get a better broad picture of what is involved with being a landlord, and would obviously also like to pick your brain about the process and lessons you've learned.

Great source of educating yourself on whatever niche you want to get involved with--especially wholesaling.

See what will make the deal sweet for everyone involved. 

I had also been involved in 2 investment properties acquisition few years ago in my in-laws investment business, by mainly helping them maintaining the properties. 

I have no idea what I'm doing but I really really wanna get involved in real estate early on!

I've never owned a rental so I'm having trouble guessing but I HAVE rented multiple house (active duty military I move a lot) and I've never seen my landlord dropping hundreds every month, usually zero dollars 95% of the time.

My concern is if the properties I am financing are involved in a cross collateral agreement that relates the apartment sale, and there is some kind of default, would the bank be able to step in (because their agreement precedes my first trust) and prevail to recover the properties that I have the first trust on?

Since your renovation in this example was under $35,000 there is another loan (exclusive) that might have helped you and is less involved than a Homestyle or 203k...same concept though.  

I would like to own multiple units (2-4) on one property (that has the ability to expand).

I suggest making copies of the dec pages of all your policies and emailing them to multiple agencies to compare.