Yeah - riding on trails you’re not supposed to ride on is a definite no-no.
And some trail systems have restrictions on when you can vs can’t ride on MTB trails (eg when wet, or single direction only trails).
I’d separate these from trail etiquette, which applies to trails that are OK to ride on. Always yield to hikers and horses. And to MTBers riding uphill. And generally, be nice.
Give way to horses and hikers, say hello to other people, stop to help if you can, or at least offer to. Don’t worry if you can’t ride something and have to walk - we’ve all been there. Just don’t decide halfway down the feature!
Always ride the correct direction of the trail - most purpose built MTB trails are one way, and one way for a reason.
And please - don’t stop and then stand in the middle of the trail!
As a beginner, can completely agree with this. For me coming from a road and then gravel background, technical mtb skills take some time.
I usually start a ride on an easier trail as a warmup and work on some skills. Then I move to more intermediate technical trails. And always finish back on an easier trails just to end on a good note.
If I had to do it all over again I would have started with a lightweight hardtail. Coming from a lightweight road bike and going full suspension on a 29er trail bike was too much bike for me as a beginner to get comfortable with on the trails my more advanced friends were taking me on - not super technical like forests but steep mountain sides with narrow switchback trails and lots of exposure. If I had to dab a foot down on uneven or rocky terrain it felt like a long way down to what I was used to with road or even a gravel bike (and I’m 6ft tall). I‘ve since sold that bike and if I got another MTB I’d probably go for an aluminum 27.5 hardtail to get more experience and confidence first.
This is one thing I did not anticipate, the bike on general feels huge!
One of the most important skills IMO for MTB is being able to decide quickly if you are going to ride a section/feature, or get off and walk.
This helps you avoid crashes, and gives you confidence to ride new challenging trails.
yeah the mondo wide bars take a lot of getting used to. First time I saw a pro woman MTB’r on an XC bike I though the bike was just way too big and it was an extra small. 
Then I got my Scalpel and the 760 bars felt way too wide. And still do.
Trim them to 750 then and see how that feels over a month or so period.
Wide bars will pull you towards the bars. Do you feel a little front heavy? Wide bars can also induce a bit of arm pump or whole arm tiredness if your upper body isn’t strong enough to hold a wider stance with your arms.
Wide bars are however very important for control and stability. So find your happy spot. Mine is 780mm (broad shoulders), yours might be 740mm? You never know until you try 
Also note that rise and stem length also play a part. Can you borrow some bars with a higher rise from a friend? Add or remove some spacers? Try a shorter stem?
I thought about one of my favorite bikes, Santa Cruz Tallboy. Then I checked the website and the price was eye watering. So get a hardtail, it’ll make you a better rider.
My 29 wheels make the bike feel like a monster truck
I think this is a solid plan! 