Hi Anne
There's no document specifically about which keys users should use to navigate in Xerte without using a mouse. By default (on any web page) users can access any form fields using the tab key and this would be the same for any input fields you add to a Xerte page. In fact in Xerte most items are reached using tab and then selected with return or space. For drag & drop activities, labels can be tabbed to, selected by pressing space bar, then tab to the hotspots & dropped with space again.
For screen readers it's good practice to mark up the label that goes with any input fields properly like this (with the for attribute matching the id):
Code:
<label for="name">Name:</label>
<input id="name" type="text" name="textfield">
There's some other good advice here:
webaim.org/techniques/forms/controls
Using variables to label things is fine as the variable markup is replaced with the string that you see on screen & it's this that the screen reader will pick up on.
Hope this helps