Travel to Greenway

If you do rent a car, you must not only book a ticket for the house but also book a place for the car at the car park.

By the way, if you get out that way, I highly highly recommend visiting Coleton Fishacre, which is a gorgeous place. I absolutely loved it and it was one of the most beautiful places I had seen. It’s not historic; as it was built in the early 1900s, but it was just very charming and lovely with beautiful gardens. And it’s not that far from Greenway.

https://www.nationaltrust.org.uk/visit/devon/coleton-fishacre

The referenced article is not so much out of date, as it was always incorrect, but that's what happens when you just Google things.
There is a ferry from Torquay to Dartmouth but it is not at all frequent (daily at best). Then you take the little ferry from Dartmouth to Greenway.
You can't do it in a day from London unless you use the Night Riviera Sleeper in one direction.
The obvious way by public transport is by train to Totnes (not Torquay) then hourly bus #92 from Totnes Station to Dartmouth (35 minute journey) for the ferry.
If you were going down the Riviera coast Paignton is the place to get off the train, not Torquay. It is the last station on the line.
From Paignton there are three choices-
A- Bus #120 hourly to Kingswear, ferry to Dartmouth, different ferry to Greenway;
B- Steam Train to Kingswear, ferry as above.The steam train stopped at Greenway request (flag) station until Covid, but the station was closed them and has not re-opened. It was a short walk from there.
C- Bus #120 to a village called Galmpton then a two mile or so walk.

There is also a cross river request ferry from Greenway to Dittisham, but Dittisham has almost no public transport. However it is a lovely village and well worth the diversion.

There are lots of places to stay in Paignton. But if you are from NY or WA states you could on a pure whimsy stay at the Roslyn Guest House, as I did last time I was there. No actual links to Roslyn, just a whimsy of mine, but a very nice.

At Kingswear most people use the foot ferry, which gets very busy when a steam train is in.
I far prefer the car ferry, a few steps down the street.

Totnes is a lovely town, a very good place to stay.
Dartmouth is a town to explore. I highly suggest walking or (another) ferry to Dartmouth Castle and the old Church.

If coming from Paignton you should stop over in the fishing town of Brixham (which also had an important role in D Day) and also take the short walk around Berry Head.

From Totnes you should also include (on a different day) Kingsbridge and Salcombe. And on a 3rd day a different steam train to Buckfast Abbey.

We visited Greenway last year- we were staying in Paignton, a seaside town an easy train ride from London, and found it easiest to book a taxi to and from the house- it was only about a 15 min ride. The house is amazing (set up as Agatha Christie left it) and the grounds are beautiful. If you are interested in seeing the house on screen before/after you go, there's a Poirot episode called Dead Man's Folly that was filmed at the property. Christie actually had her own home in mind when she wrote the book that episode was based on, so it nicely aligns with descriptions in the story.

Two place for food nearby, which we looked into but did not eat at are Sharpham dairy/vineyard, where you can taste English wine and local cheeses: https://sandridgebarton.com/select-a-tour/

And Riverford Field Kitchen, which does organic vegetarian food: https://fieldkitchen.riverford.co.uk/

If you're spending time in the area I would also visit Dartmoor- beautiful landscape great for walking or visiting small villages.

By the way the National Trust website says that you do NOT need to pre book your visit if arriving by green transport (that is, anything but driving there) and that non members get half price admission if arriving after 2.30pm.
In fact you can only pre book if also booking a parking space.
The afternoon half price is new this year.