B: "Layouts and default_pages are not correctly migrated"
The march commit is already part of plone 5.1.2 (p.a.contenttypes 1.4.10)
So making a CMFPlone release using 1.4.11 would not change anything (would not make the situation worse)
C: Link CT broken:
Is broken in 5.1.2 (p.a.contenttypes 1.4.10) so making a CMFPlone release using 1.4.11 would not make the situation worse.
If we use the already released 1.4.11 of p.a.contenttypes we do not make the situation worse.
If we release and include 1.4.12 we do make the situation worse.
Of course unless we can fix these three issues - but I suspect it could take time.
A) I have no idea. A migration would fix shared layouts, but not layouts stored on filesystem in add'ons. After this issue I learned out that mosaic layouts are non trivial and broekn anyway as soon as you use anything 'non' transient in p.a.mosaic 2, so I'm not sure if this really is a 'big' issue, but it's still annoying and might be a red flag for more knowledgable developers where the Marker Interface clean up/fixes might cause problems, I don't know.