1. The Summon Negation Window starts as soon as you place Ingunar on top of the Materials. Note that this is technically before Ingunar is on the field. A monster is only considered to be on the field after its Summon Negation Window is over.
2. That's correct. The effect that starts a chain would have to be negated as negating a Summon performed during an effect's resolution would interrupt the resolution of a chain, which is not a legal action.