In short - "Stem mastering" is mastering from the stems of a mix - Instead of being presented with a stereo mix, you might get a drum mix, guitars mix, vocal mix and effects mix, all at unity gain. It lets you tweak in a more "microscopic" fasion. Some mix engineers like to have it done that way, some don't.
But as long as we're getting into semantics, the "actual" term for the preparation of a production master disc that will wind up being the final version before a glass master is made is called "premastering." Although, the "accepted" term has been truncated to "mastering" without much argument.
I agree on the engineer's ears and years over the gear, but there is also absolutely no substitute for a quality monitoring chain. The best engineer and /or gear in the world is worthless without a quality monitoring chain to hear it properly. It's essentially impossible to make stereo field adjustments efficiently using headphones. An "open air" listening field is an absolute requirement. The same sound needs to hit both ears at different times. With headphones, that can't happen.
The only time I could ever imagine anyone using 12 plugins, 31 EQ nodes (or even 10 for the most part) and "hours" on anything is if you're trying to master your own mixes. One of several reasons why it's generally a very bad idea.
Try to never aim for a particular RMS level - Every track will "ask" for a particular level. Some want to be loud, some don't. Aim for a particular "SOUND" and worry about volume later. This is why most commercial releases sound so damn... Deep breaths... Deep breaths... Sorry. Almost lost it there.
Anyway, your files sound fine for the most part. The "fuzzy bass" that comes in around 0:35 thru the end has an awful lot of high-end energy in it, but that can easily be chalked up to the mix and artistic license.