They've said they won't retcon the ritual, and Leliana being undead will be explained.
As with most things, I'd like to see some three different responses per given choice. Let me see if I can remember what they were.
1. The ritual occurred. Maybe word got out, maybe about the boy's very powerful father, or about the ritual in general. Maybe word gets out about the ritual after hes kidnapped, but he's kidnapped by an interested party, and one must help Morrigan recover him.
2. The ritual did not occur, but the warden and Morrigan had a relationship. Word gets out about the boy's very powerful father, and kidnapped by an interested party and one must help Morrigan recover him.
3. There is no child. Morrigan's plans need to take a detour. Perhaps she needs an important relic, like Flemeth's staff (not her broomstick from Witch Hunt, but the staff she'd actually use in combat, if she needed one), or something like that. One must help Morrigan find the relic she seeks.
In all three cases, there'd be a chance to side with Morrigan at the end or someone else.
Witch Hunt doesn't really matter if you sacrificed yourself with no DR. Why would an Orlesian Warden who's never met Morrigan want to know what she's up to? Other then messing with old magic mirrors why does a Warden that Sacrifices Alistair or Loghain care what Morrigan's doing?
Perhaps the warden told Alistair or another survivor what Morrigan proposed (I headcanon otherwise, unless my warden needs to recruit Alistair or Loghain for the job) and word got out to the wardens, who wanted an explanation?