We're talking about adapting to game flow here. If 4 players often limp/call raise - of course you limp behing because you raising does not push them out of the pot. This is valid for any limit.
For example, when I 6 table, on one table my stats look like 27/22/7, on another 13/5/0 because a maniac or calling station is sitting to my left. No use raising those unless you have a hand that flops good. I believe that you always run into those station tables so your arguments are of course valid. But at least in PLO25 there are quite a few tables where you raise and take it preflop or get one caller and c-bet to win it.
In holdem flopping middle pair is quite good, in PLO it's worthless, so it's relative. Also depends on reads - if villain cannot let his trash go - fold your missed hand and get max profit out of made hand. C-bet with air is worthless vs several opponents, that's for sure.
I'm not arguing and postflop play being superior to preflop in PLO, it's obvious.
1) Raising situations. I raise about 13% avg. of my hands. If the table is normal/passive, I raise like 20%, if a maniac/station is present, I raise way less. I raise with hands that can flop the nuts obv. Button open raise is wider of course. Again, it's hard to say anything about the lowest limits, I bet it's fit or fold in those, but in a bit higher limits raising preflop eliminates at least 3 players (relative).
2) It is not wrong to limp, but then you will NEVER have initiative in PLO which is immensely important. I suggest you observe some higher stakes tables where a guy with 80/50/20 stats is owning the whole table. You think he's hitting all of the hands? Hardly, he just has initiative so you have to play fit or fold against him. Lets say you hit top pair vs him - are you ready to all the way with it with no real draw, and he's pounding the pot real hard?
I raise to get initiative - if villain misses he has to fold (most of the time, unless he wants to make plays), I don't have to fold if I miss.
When you limp and hit a monster, you will hardly ever stack your opponent. Having a wide raising range, hitting lets me win a stack easier because villain knows I have a wide range and stacks off with two pair for example.
Again, if your usual table is made of bad players who cant fold - sure, limp more, raise less.
P.S. You get alot of HU situations in a bit higher limits if you raise.