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Super simple here. Can Legions of Nagash armies still use the Battalions from GA Death, as long as they have all the necessary units to do so?

No. They can only use the formation in the codex.

Yes. Legions of Nagash army can use units from GA Death but only as allies (with allies limit points) and those units won't get Legions of Nagash allegiance (bonuses).

This is where the 'allegiance' of battalions gets a bit vague. RAW, a battalion must have the given keyword to be part of a given allegiance. But battalions haven't ever been treated that way, they've been treated as if they have any keywords that all the units in the battalion share in addition to the one listed with the battalion. AFAIK that latter part isn't written in the rules anywhere (though I could be wrong). Without such a rule, then RAW no battalions outside of generic Order/Chaos/Destruction/Death can be used since they don't have a grand alliance keyword (and all matched play armies must belong to the same grand alliance). Even assuming an unspoken grand alliance keyword specialized factions could not use generic alliance battalions because no faction lists a generic alliance as an ally. In this case, the Grand Host of Nagash does not have "Death" as an available ally and could not technically use "Death" battalions like Deathmarch even if what I described above was set aside.

All this touches upon a larger issue that Age of Sigmar doesn't function very well o(r even at all) using strict RAW. There hasn't been a single AoS tournament that has actually run strict RAW (even putting aside any house rules present). And the vast, vast majority of players don't either (though I suspect barely anyone actually realizes it).