Arzja

This isn't allowed anymore btw. The core rules prohibit duplicate mount traits.

Just want to point out. You can't actually end halfway down. Jumping down can't be done if the distance is greater than your move characteristic.

Pretty sure the Gaunt Summoner of Tzeentch can just be taken as a part of a Tzeentch army. No allying or coalition needed.

The core rules have been available for about a week now on Warhammer Community. It's definitely still in. Like I said, just a wording change. The old wording said re-rolls happen before modifiers are applied and the new wording says modifiers are applied after re-rolls. Functionally the same but I can see how people get confused.

Modifiers were always applied after re-rolls, the wording changed a bit. As for the scenario in the question. The cast technically failed. So your friend would be able to re-roll if he chose to but due to the modifier it actually succeeds. So your ability doesn't apply.

There's the problem. The Khinerai ability doesn't trigger immediately. It just says you can roll a dice after all attacks have been resolved. So that means it occurs at the same time as wound allocation and since slain models are removed during wound allocation due to wound allocation technically happening one wound at a time, both abilities are essentially occurring at the same time. Which is usually resolved by turn order in other conflicting rules. A small note that may cause even further confusion. The wound allocation section of the rules does actually use the wording "Once all attacks have been resolved...." which could be read as a synonym for immediately but that's debatable. Honestly GW needs to clean up their language to prevent things like this. Some of the previewed rules for AOS 3.0 do seem to use better language so fingers crossed I guess.

The retreat move also occurs after attacks have been resolved though. So it may very well be dependant on whose turn it is.

Don't forget that the Doppelganger Cloak has been FAQ'd to only be a once per battle. Still strong if you use it at the opportune moment and as a deterrent to using the locus on that hero but not quite as strong as it was.