it would have to run on top of XMir, which is IMHO highly undesirable for a core app.
Remember, they were planning on doing just that - running the entire desktop under Xmir for at least one release cycle (in 13.10). They persisted with that idea for something like 4 or 5 months before eventually giving up on it as a bad idea...
This is quite similar in concept to Android Activity/Fragments.
My fear is that those apps while looking good in phones and tablets won't look that good, in larger screens, with reduced functionality compared to traditional desktop apps and with too much empty/padding space, specially in larger laptops and desktops. The file browser they where working on is an example of this.
One of the goals is to use different UIs on different form factors. The reason the file browser looks the way it does (including the reduced functionality) is because the desktop part of the equation has yet to be finished. You can see here that they are aware of the issues and looking at how to best to address them for the desktop.
If you go to 50:14 in the video you can hear specifically about a tool in the sdk for changing layouts to affect things like spacing, so basically the desktop design is still being fleshed out and I wouldn't worry about how the mobile apps look when used on the desktop just yet.
Unity 7 on Xmir is probably going to happen for Ubuntu 14.10, just to make sure that XMir actually works for all apps and as a backup DE. Depending on the speed with which Unity 8 Desktop can be finished, it may or may not be the default for 14.10. The earliest that Unity 7 won't be present is almost surely 15.04, but it just depends on how well the Mir/Unity 8 Desktop is working by 14.10. Full convergence won't happen before 15.04.