Good idea indeed! I never seen this be used in a timeline, but I doubt that Russia would be able to occupy almost all of North Korea before China would also step in, so a joint Chinese-Russian occupation of the DPRK seems like the inevitability in a scenario like this.
And despite the tiny DPRK-Russia border, Russia could simply
conduct amphibious landings in Korea like how they did against Japanese-controlled Korea in WWII, only that this time the Russian army is repeating history against Kim, instead of Japan.
I also wonder how much popular
Brian Reynolds Myers' takes that North Korea's ideology is a repackaged version of Japanese Showa-era Fascism would be ITTL, probably much more, since leftists worldwide would want to completely disown the Kims' North Korea at all costs.
Or utilize VDV paratroopers (
https://en.wikipedia.org/wiki/Russian_Airborne_Forces) to land the flying Spetsnaz to major cities from air in orser to hasten the (Sino-)Russian occupation of North Korea before American nukes could make it in time. If the operation is launched after the nuking,their forces would need to wear gas mask and other similar equipment to not contact the radiation. If it goes badly, it might become Pristina Incident 2.0 where NATO and Russia confronted in Kosovo over occupational dispute that almost caused WW3. ->
https://en.wikipedia.org/wiki/Incident_at_Pristina_airport
You bet,almost all of TTL World (except extreme Tankies, NatBols and few Fascists) would view Jucheism as Showa Statism painted red, making OTL Homefront game not so abnormal.
In any case,if Russia intervenes, this would set a major precident in relations between Russia and America, for better or worse.
I don't think an assassination attempt would be very successful but I do think that China or Russia would be in a position to intervene militarily and rush to capture key personnel and positions.
North Korean forces are pretty "occupied" at the moment dealing with the counter invasion and defending the Kim regime which could leave the borders with China and Russia weak.
China might prefer to have a buffer state in the Korean Peninsula even if it's a rump state that they have to govern with a close eye.
After the nightmarish death of Jang Song-thaek,no high WKP official nor any Kim family member will dare hint the mere suggestion of a coup against Jong-il,let alone actually conducting it.
To me, China would take west North Korea while Russia the eastern portion, and the ruined Pyongyang would be divided between the first two powers, America, Japan and South Korea akin to Berlin post-WW2.