there is something called a burden of proof, and it basically states that a lack of evidence to the contrary does not affirm something.
So, basically, because it isnt stated that wolverine isnt immortal, or because he hasn't been killed permanently, does not mean he is immortal.
The burden of proof lies in the idea that if you make a claim you must back it up with evidence that supports your specific claim.
Applied to Wolverine:
really simple, is he or is he not immortal. it is plainly obvious that EVERYONE here agrees that he is not immortal. Immortal means cannot die (it has nothing to do with old age, that is called eternal youth. immortality means litterally not mortal).
The argument you are making is that he HAS died, then come back. Thus, not immortal.
I also don't see much problem with the regeneration from death thing. Without going into a deep debate about the nature of the self and the "soul", if wolverine's cells have mutated to all be like stem cells and replicate + assign new roles to new cells, it works. In fact, its one of the few mutations that real world science would actually be able to support.
When the cognitive functions of the brain stop, the individual cells of the body still live on until they are deprived of nutrience, so while Wolverine may be dead, his cells are not, and they can repair any damage that caused the death + restart cognitive function.
HOWEVER, all that I have typed above is speculation, and is in no way supported by comic book evidence. I am mearly wondering why people think it is outlandish for wolverine to have this ability...
PS: In reference to the regeneration feat in no 43, it would NOT be possible given the circumstances hypothesised above. No organic matter would have survived heat intense enough to melt flesh from the bones. Any organic material contained inside the bones would turn to carbon ash from heat transfer through the METAL.
I'm more inclined to say it was the artist + new writer colaborating to make the story pack a bit of an extra punch, physics and biology be damned. It works, and im personally glad they did it (this was the first issue of wolverine i have EVER read btw) as it did make for a cooler plot, though I still think regeneration from 0 organic matter for wolverine should be looked at with skeptiscism, as I don't believe the evidence for THAT has been shown (though regen from some/any organic seems to be verifiable)