There are other folks on the team who can answer more of these questions than I can, but I'll do what I can. :-)
(If you email the
support ninjas, you're guaranteed a response from the most knowledgeable folks, btw.)
If a line is selected, double-tapping adds a midpoint. If the line isn't selected, you will get a label that's close to the line, but not attached.
To attach the shape, drag it to the line, then tap the line with another finger. It's currently somewhat hard to do this, because the iPad simulator didn't support multi-touch. Revising actions like this is one of the things we're working on now that we have access to actual iPads.
To select text within a label, select the it with a single-tap, then double-tap. Once the keyboard slides out, the context menu will apply to the text in the label, rather than the object itself.
The app tries to move the selection out from under the keyboard, but there are apparently edge cases we're missing; Screenshots and/or sample documents sent to the support ninjas would be really helpful in getting that fixed. (Screenshots on the iPad work the same as the iPhone - press the power and home buttons simultaneously to add them to the Photos app.)
Hope this helps! Thanks for purchasing - we really appreciate your support!