Last night Obidos before 11 am flight?

I wouldn't do it, too nerve racking.
I like to be quite close to the airport for an expensive transatlantic flight, especially since it's an AM flight.

Are you sure you can't change your itinerary to put Lisbon at the end?

Can you give us more information about your itinerary and how far along are you on your reservations. You might get some helpful suggestions. For myself and my husband, if flying in and out of the same city we try to leave all of our nights in that city until the end of the trip. Depending on what time you arrive and where your first stop is, if possible you might want to go directly there and leave Lisbon for the end. That might free up one night for Obidos.

right now I am pretty close to finishing my itinerary with reservations etc. so changing things might be hard. this is what I have right now. We are working around a couple holidays April 25th and May 1st so that's part of the reason it is set up this way.

Lisbon 4 nights (arrive 6 pm first night so didn't count that as a real day)
Day trip to Sintra on 2nd day.
Pick up rental car.
Visit Tomar on way to Coimbra
Coimbra 3 nights
Douro Valley 3 nights (vacation from our vacation and anniversary relaxation time, I know folks do this in fewer days but we want an extra time to relax)
Porto 3 nights
1 last night -- I originally thought we would stay in Obidos, but got stressed so booked it 1 night in Lisbon.

It might be a lot of work to move all this around as I have both hotel and some tours already booked.
but I am open to input anyways

Do you think I can reduce my visit to Coimbra by 1 night?
that would leave 2 nights on the other end of the trip for maybe Cascias. Is Cascias close enough to the airport for us to drive ourselves there in the AM?

To answer your last question, yes, you could easily drop one night in Coimbra. Do a tour one day and stop in Conimbriga on your way into town.

Living in Cascais, I can attest to it being quite easy to drive to the airport for your flight. The biggest issue is allowing time to turn in the car. The traffic gets bad between 7:00 and 8:00. Even though it sounds like a lot of time, I would likely leave around 6:30 to beat the traffic and have plenty of time to turn in your car.

Obidos haș a nice hotel it is like a Spanish Parador. If you do stay there, you would need to depart very early in the morning.

I have not done Obidos to Lisbon. If it were me, I would do it. According to Google Map it is about a onehour driving time

We stayed our last night in Lagos, drove 1 hr to Faro to catch an 11:25 am flight home. The other couple travelling with us took 3 hr train ride back to Lisbon to catch a flight home at about 1:30 pm. You just need to factor in extra travel time just in case. I would rather spend night somewhere interesting than just a airport hotel. Good luck.

We took a town car from Caiscai to the airport before our noon flight. The driver wanted to leave at 7 because of weekday morning traffic. And there was bumper to bumper traffic but we got there right before nine. I guess you also need to take into consideration time to drop off your car. Maybe someone can advise how far the car rentals are from the airport.

We stayed in Caldas da Rainha, not that far from Obidos and drove our rental car in to the Lisbon airport for a 10:15am flight. Our rental car company opened at 6:00 and we were there shortly after in an uneventful hour drive. All went well. We actually got there too early as our flight gate wasn’t posted yet.
After you get your bags checked (if you have them) and are ticketed make your way through the carry on scan. You will get to duty free land and a food court. Don’t stay too long because you have to go through the passport check. There is where the stoppage lies. Early on there are too few agents for the non-eu people. As the lines get longer they add more agents but people in a hurry get stuck.