How can I enable adoptable storage and get my Samsung Galaxy on5 to use my external SD card as it's Internal Memory? I may have @ the time I put my SD card in originally but I don't recall ever coming across that feature/option. Also, I definitely do not wanna erase/reformat my 128 GB SD card (that's about 3/4 full) to be able to see if that's is/was ever given as an option when putting a brand-new card.

If ANYONE'S able to offer me ANY advice/ideas, tips/tricks, choices/options on this matter as well as ROOTING and/or UNLOCKING for my Samsung Galaxy on5 SM-G550T2 phone.
You can use the SD card as both internal and external storage without rooting.

Infact there is already and answer here in SE in how to do just that:

https://android.stackexchange.com/a/145458/127015

I would strongly recommend you backup your SD card, just in case.