"Tributemonsters from your hand or field, and/or banish "Nekroz" monsters from your Graveyard" and "Ritual Summon 1 "Nekroz" Ritual Monster from your hand whose Level exactly equals the total Levels of those monsters." are not considered to happen simultaneously.
The "If you control no monsters: You can banish this card and 1 "Nekroz" monster from your Graveyard; add 1 "Nekroz" Spell Card from your Deck to your hand." effect is activated from the Graveyard and it starts a Chain Link. It does not target. Banishing this card and 1 "Nekroz" monster from your Graveyard is a cost to activate this effect.)
While "Imperial Iron Wall" is active, you cannot banish monsters, but if you can Ritual Summon by Tributing monsters from your hand or field, you can activate "Nekroz Mirror".