Can GC show the average speed and other relevant data of a selected graphic segment of an activity? When I zoom in on a segment, I still read the average speed of the whole activity, and not of the segment I'm interested in.

Not the best forum to ask this question. That feature cannot really be added by a Connect IQ app. And although not directly available in Garmin Connect, there is a workaround - if you want to see stats for a specific section, you can use the Trim function. The trimming is reversible, so once you review the stats you can rest the original back.