We're riding the GAP in late June and have been watching the Pittsburgh construction closely, I believe it's slated for an April completion date. Check out the GAP facebook page for latest updates, it's much more up to date than the ATA site. There are pictures of the ongoing construction around Sandcastle and its nice to see evidence of progress.
While studing Google maps with the bicycling directions turned on I see an alternate path to Point Park that runs along the southern shore of the mon river. Using street-view in Google maps it looks like you can cross the bridge right next to the park and ride directly the park, thus avoiding downtown Pittsburgh. I have not been able to confirm this in any way other than observing cyclists captured in the street-view photo's so I don't know how official this alternate connection is.
Keep an eye out for posts from user Spinaker on the forum, he's very knowledgeable about the Pitt end of the trail and is very helpful.