Both Mallet and Pineault were HOST PICKS, and neither would have been able to get a GP event by any other means. Mallet is not on the SB list that is used for GP selection (it's the 2017-2018 SB list) because she didn't compete last season. Alicia Pineault is on the SB list, but at #88---way past the #75 limit for GP selection. They got those spots because they were replacing two ORIGINAL host picks. Any host is entitled to replace host picks with its own skaters, and there are no eligibility requirements for host picks.
In some cases, a host doesn't have a suitable replacement within its own ranks, so has to go outside to find a replacement. This can happen with Pairs or Dance vacancies if the host isn't strong in those disciplines, but Canada is strong in all disciplines and didn't have a need to look elsewhere for replacements.
BTW, there is no rule that a host has to choose a skater who already has one event over a skater who has none.
Of the ladies on the substitute list, Skate Canada can choose a US skater since currently only one US skater is on the SC roster, and that is a consideration, given limited time and travel. Craine and Lim both train in the US, so they also meet travel restrictions.
Japan is maxed out on US ladies. They may wait to see the SC choice before naming theirs.