I've never heard anyone say just mālama. It would be mālama pono or e mālama pono.
In my limited experience, I agree with this. I have seen/heard "mālama", "mālama pono", and "e mālama pono" all used in the same way. I believe all three should be acceptable.
ʻAʻole. He hewa keia manaʻo. We never say just mālama, my grandfather would slap us for taking shortcuts in the language. Why should it now be acceptable?
It depends on the context. Also, this seems to be done sorta new way, like it's leveling out the language. And there is alot of slang and adopted words we probably won't see here.
Here's an in depth digital book about what they've found about the language.
I'm not an expert, if you know the language well take a look and tell me what you think. I haven't read it all.