GW staff don't know what the mail order staff can and can't do - ring the mail order team and ask them direct.
As to a Wych Lord, that's an Archite (Archon = male Lord, Archite = female Lord ... er Lady ..

) Model wise there is not a specific Archite - the
GW store only has (and has ever had) 2 Dark Eldar Lord models listed at present as an Archon and a Dracon. You can certainly use your existing Lord figure as an Archite/Dracite if you wish.
You can have either an Archon, or an Archite - but you cannot have both in the same army. Taking an Archite, or having a Dracite lead your army (which also means no Archons or Drachons, as per the sidebar on that page) allows access to a different
FOC structure (Wyches as troops, warrior and raider squads are elites). A Wych Lord has Wych weapons as standard, but can be armed with anything from the armoury just like an Archon/Drachon, but can also take Wych retinue (and cannot take a normal Darkl Eldar retinue).
I don't understand the last question though. What's not complete about your Wyches?