First, to set the stage, a 1219C2 and a USN MK2 are the same knife. The only difference is the branch of service stamped into it - 1219C2s have USMC on them. USN MK2s have, duh, USN MK2.
TECHNICALLY, the post-WW2 versions are the MIL-K-20277? where "?" is the change revision letter and these knives have no service branch designation. I still call them 1219C2s for simplicity sake. "Everybody" knows what knives 1219C2 or USN MK2 refers to. I'd bet not 10 people in the world call the "modern" ones a MIL-K-20277, and all of them are bean counters for the government.
Being simplistic about my answer to your first question is, Yes/No/Kinda/Maybe. The 1219C2/USN MK2 is not a "style" of knife.
The 1219C2/USN MK2 is a knife made to specific requirements for sale to the US government during WW2 for which 5 companies were "approved" to make the knives. (Boker was approved but never got a contract). Union Cutlery was one of the 4 companies awarded contracts to make the knife during WW2. UC decided to stamp their knives "KABAR". They could have stamped them UNION or UNION CUTLERY (or if they wanted to bring back dead brand names such as OLCUT or KEENWELL).
The MIL-K-20277 is a knife made to specific requirements for the US government that is a tweaked version of the 1219C2/USN MK2.
Specific knife popularity among collectors is one of those ephemeral bits of knowledge. Some people only collect KABAR brand 1219C2s/USN MK2s. Some people only collect WW2 versions from any manufacturer. Some folks collect anything remotely resembling the original 1219C2. There might be collectors out there who only want WW2 1219C2s that have red spacers.
Why is it that the entire genre of WW2/VN era/post VN era 1219C2/USN MK2/MIL-K-20277s are called "KABARS" regardless of who made/makes them? Good question. The answer is, in reality, no one knows for sure.
One tale is that Kabar supplied the first batch sent to the Marines at the Marine training depots. (I read somewhere that Camillus' first batch/batches went straight to the Navy.) When the DIs were training the new recruits about their "new fighting/utility knives", the first step was to instruct the boots what the item of the day's training was.
First, imagine a DI standing up in front of a class of newbies holding up a 1219C2. Now, imagine the DIs' spiels went sorta like
"This is the 1219C2 Fighting/Utility knife, made by Kabar. It is 12" long and has a 7" blade, with a stacked leather disk handle. ............"
Marines being Marines, after a few classes, the knife nomenclature would get shortened from "1219C2 Fighting/Utility knife" to a shorter version, "the Kabar" or "my Kabar".
What's a Marine gonna say?
"Where's my 1219C2 Fighting/Utility knife?"
or
"Where's my Kabar?"
When the knives started coming in to the Marines from other companies, were they gonna be picky and ask
"Where's my Camillus?"
or
"Where's my Robeson-Suredge?"
or
(shudder) "Where's my PAL?" ???
Nope. Everything was a Kabar.
And the world is a better place for it.