Newly retired couple's 1st trip to Greece, only 1 week before we travel to Italy.

I would strongly consider choosing one or the other, as then you could get to all on your list for Greece. Plus, it's a long flight, so make it worth your while. I love Italy and have been there 8 times more than I have been to Greece, but especially if you want time to explore all of the little churches, take the two weeks--you will not regret it!
If you stick to a week for each and want all of those mainland spots to be included, I would skip the islands. (The mainland has gorgeous coastline too!) You can visit Nemea instead of Olympia to cut down on the distances--did you plan to rent a car? May was a lovely time to be in Greece.

With Greece, you definitely shouldn't make a list of "must-sees" taken from travel guides or other tourist magazines and websites.

First, you need to look at a map, understand that public transportation is sometimes nonexistent or only slow buses, and that all the "must-sees" are far apart.

In a single week (6 full days) with a rental car, you could go to Nafplio, possibly Delphi if you optimize your travel time, then stay two or three days in Athens. That's it.

Sorry, if you thought you could do better than that, you should have added a week in Greece.

Note that if you manage to include Santorini in your plan, you may find direct flights from Santorini to Venice.

You'll want a rental car of you're traveling the Peloponnese. If you travel all the way to Olympia, you could do a car drop off on the west coast, like Igoumenitsa and ferry to Corfu. Then later take an overnight ferry, or cheap flight, to Italy. This will allow you a one way trip, instead of a round trip back to Athens.

I'm not sure what the ferry schedules to/from Corfu are like in May. This route will take extra care to coordinate the car rental and ferries.

You could do a car drop-off on the west coast, like Igoumenitsa.

Note, however, that renting a car in Athens and dropping it off in Igoumenitsa with the few rental companies that allow it will cost around €300/day for a small car (about 10 times more expensive than a drop-off at the same office as the pickup).

When you say two or three days is that one night, two nights or three nights? It’s best to count the number of nights you’ll be there not days.

If you go to Santorini, fly from the US to Santorini. When checking airfare look to see if it’s best to fly to IT first, etc.
Santorini does have sensational sunset views depending on where you are at sunset. If you can afford to sleep in Oia do so. If not, sleep in Fira. Make sure you visit both Oia and Fira while there. If you go to Santorini, don’t bother going to Hydra which is a convenient day trip from Athens.
Nafplio is where the Greek locals go on the weekend. Nafplio is also the springboard to visit Epidavros.
You need three nights in Athens unless you add a day trip to Hydra then make it four nights and skip Santorini. Allow three nights for both Santorini and Nafplio.
You can fly between GR and Venice / Florence. You need a minimum of three nights for both Venice and Florence.

Really ought to pick just one, or add another week.

"Two weeks", however many nights that is, turns the trip into a 'check list' vacation. And one day will be devoted to moving between Greece and Italy.

If you do choose to go to both, book a multi-city (aka open jaw) flight plan.