Thank you Palal and Hornetcoach for the informative responses.
I've decided to take the 20h00 flight to OPO. (I really dislike rushing through airports; too many potential delays and obstacles). My party is concerned about three flights in a row, but we'll just relax in Porto for a day. I'm really visiting to explore ex-pat opportunities which I've heard are quite good in Porto (actually I've heard this about Portugal as a whole).
I am now thinking we'll rent a car and drive from Porto and return it at LIS before heading into the city center (with an Uber). This gives us the flexibility to perhaps do what Hornetcoach did and stay in or near Obidos, tour the area during the day and enjoy the old town after the crowds leave. Plus, I've checked with Avis and even with the drop off charge, it's less expensive than 4 RailPasses, though I haven't priced individual tickets (btw, for those who may be considering the RailPass, it includes a pass for Lisbon's interurbanl lines; I'll be looking for those as a seperate purchase to use in the city).
I've been traveling to Europe for decades now, but Portugal was one country I've never managed to visit. I'm really excited to experience it.
Great idea! I'm booking a night at Posada Castelo de Obidos. Thanks for the nudge!