Doug,
I don't know that area all that well, but you might have soejm problems with your maps.
MapQuest shows McCaslin Dead ending and Enlow dead ending. Betwen them is an interchange for the airport that is maybe 10 years old. Maybe the map you are using is older?
There is a map for the Montour trail that shows a "Planned airport connector" through that area, so there might be a stealth way to do it now.
http://www.montourtrail.org/maps/cora.html
(Note, this map also shows Enlow as dead -ending, but the map is incomplete as to roads)
I'd consider taking the shoulder of the Airport expressway east to the McClaren exit, then S-SE on McClaren to the Montour Trail. I've never ridden on the should of that road, but I've looked at it from the window of the bus and decided that it might be do-able.
Then, when you get to Coraopolis road (pronouced Corr-ee-oop-oh-lis, BTW), you might want to consider going west to Ferree St, then NE over the Neville Island Bridge, SE on Grand Ave, SE on Neville Rd, and across the Fleming Bridge to Island Ave. This would save you a very large hill with ugly traffic. Possibly uglier traffic than Grand Ave.
***
Now, if it were me, and I didn't have a chance to get any more information than I have now (I've never ridden on the Montour trail, for example),
I would do the shoulder to McClaren, go 100 feet south on McClaren to these roads (that I DO NOT KNOW)
Aten (Starts NE, then goes) SE 1.5 mi
Cliff Mine SW 1/4
Enlow SE (Saying "WTF?" So am I.) 1/2 mi
That brings us to roads I've ridden on - only once, but not too long ago. Part of Oscar Swann's Potato Run Rd ride.
Stubbenville Pike E 1/4 mile
SHOULDER OF US 22-30 (Ugly) E 1/2 mi.
Campbell's Run S 3 mi
Baldwin Rd NE 1 1/4 mi
Stueben St E 4 mi
Down a huge hill to the west end circle -Ugly (note capital U)
Get under the rairoad overpass and down to the right onto
W Carson St, E 3/4 mi
Left at light on
W. Station Sq Drive E
A Bike Trail ( ! ! !) starts there)
Follow the bike trail 1 mile to Station Square (Yuppie shopping site)
You can take the poorly marked shared use sidewalk accross the Smithfield st to Dowtowm (pronounced Dahn-tahn)
I'm putting this up partly in hopes of someone more knowledgable putting a better route.
Good luck. Please let us know how it goes. I would be delighted to buy you a beer, if you survive.
Mick