Those are all a little dated, and are from the times where people used T-9 (basically typing with the 1234567890 keyboard, where each number has 3-4 letters on it).
Now, in the age of smartphones and autocorrect, it's much less frequent to see this type of slang. I'll add precision on each one of them.
(0) mdr, dsl, a+ and others are almost always written in lowercase. They're still common (except cv and pdp).
(1) That's not really true, "never used" is too strong. It's not an effort at all to use apostrophes now. I'd say they're used more often that not.
(2) That's just not true. It's hard to understand if you never use accents.
(3) Used to be true, but now it's almost only used sarcastically (or by pre-teens maybe).
(4) No, people don't really do that anymore. And I don't think I've ever seen ier or parl.
(5) I knew like 2 people who wrote it that way, and now it's considered kind of childish and annoying.
More generally, all those were tricks to type faster. Now in order to write like that you'd have to fight against the autocorrect, and you'd be slower than writing normally.
Some of them are still used, like jsuis/chuis, mtn for "maintenant", etc., but "langage SMS" as it used to be is not really a thing anymore.
And about "sounding more native", I don't recommend using them until you're entirely comfortable with them. There's nothing weird with someone writing normally (provided you're using a casual register), but it feels a lot weirder if you use slang incorrectly, and you'll instantly appear as a non-native. So until you've spent a lot of time speaking to young people, and figured it out naturally, just use correct French :)