Sharonov, I've been thinking about your post. We've spent some amount of time in coastal Cornwall the last two years, working on plans for a new tour we're launching in April. Last year (late September into October) we did a 100 mile walk around the western tip, from St. Ives to Falmouth, and then made a side trip for a few days to the Isles of Scilly. We did take a few rest days. (We met up with Pauline and Steve several days.)
The coast is rugged and spectacular! But there are definitely hills. The villages tend to cluster around harbors and then rise up on the hills that surround the harbor. I've been trying to think about where you could base that wouldn't involve many hills. St. Ives is beautiful (we stayed two nights in 2016 and again in 2017), but both years our B&B accommodations were uphill from the harbor where most of the shops/restaurants are located. The Tate Modern museum is uphill, and so is the train station. (It's possible to take taxis if the hills are too much.) So I think it depends on how much of a hill would be a problem for you.
The St. Ives train station is at the end of a separate line... maybe a 20 minute ride to the station where you catch the main line that goes from London to Penzance. So it's possible to use the trains. However, when you take the train to Penzance, it will be an uphill walk to visit much of the town.
We did see buses along the coast during our walk in late September/early October. I'm not sure when they stop running. But you'd have to plan your itinerary carefully around the bus schedules.
You might also consider Mousehole (a very picturesque village where we've stayed several times) or Marazion (where Pauline and Steve stayed... small but flat and right by St. Michaels Mount-- which also involves an uphill climb. We stayed in Marazion two nights each in 2016 and 2017. I was also thinking about Falmouth for you, which we loved, on the Fal River. It's a bigger place and more flat, but I'm not sure about transportation and you'd have some amount of travel to see other parts of Cornwall.
If you really want to see Cornwall, I was thinking you might want to book a private driver for several days. It will be expensive but you'll be able to see much more than relying on public transportation which isn't always flexible or efficient.
Kathy