Is this also the case in Gen III then? The Legendary/Unobtainable Pokémon aren't necessary for completing the Pokédex such as Lugia and Ho-Oh.
Things would be so much easier if we could trade with Gen II games... ¬.¬
That's correct, Ho-Oh/Lugia and the Event Legendaries are not needed for 3rd Gen and D/P/Pt's National Pokedex completion.
Sadly you cannot transfer from 2nd Gen to 3rd Gen, since the GBC is incompatible with the GBA.