When attempting to attached a picture using the camera, there are a few problems:
On my Note 4, after I select add picture from camera, I can swipe up in the
camera area and it makes the viewfinder full screen. Also, in the top
right there is a pic of a camera with arrows in it - this lets me change to
the back camera (i.e. not selfie mode).
Not sure about your 3rd question as mine always works fine.
Hope this helps!
@DobieG, my phone, Nexus 6p, behaves the same as yours. Yes, you can click the camera button to switch from selfie mode, but why can't it be set as the default or at least remember the last mode? I can also swipe up to see the full camera on the screen. However, taking a picture still cuts it off. If I first take a picture with my camera app and then attach it to the text, it's fine.
It sounds like you might be using an earlier version. The selfie camera bug has been fixed in version 6.2.2. Please update if you have not.
If you are already using 6.2.2, please uninstall the update. Then, reinstall it fresh. Hopefully this action will fix the camera bug.