I walked the c2c a couple of years ago. In terms of using an American credit card you will be fine with either Visa or MasterCard, some small places won't take American Express as they charge too much commission. You'll be able to use these in ATMs but I expect there are hefty charges for taking out cash with a credit card. There certainly are for UK credit cards. Your regular current (checking???) account card will be better to use.
In terms of ATMs along the way:
I can't remember if there is one in St Bees but as it's the start make sure you turn up with cash.
In my experience the shop in Cleator never has pies.
But it did have an ATM when I passed through 4 years ago.
The next ATM you will hit will be in Grasmere. Youth Hostels will take cards for accommodation and food and drink. As will pubs. After that you have quite a good run with machines in Glenridding (inside the village store) and Shap. Kirkby Stephen is a decent sized town and has a bank and an ATM in one of the supermarkets. There is a gap then to Richmond which again is a town with a handful of banks. It's after Richmond that it gets a bit sparse. I left Richmond with a good chunk of cash on me. There might be a cash machine in the shop in Reeth but I can't remember for sure.
Unless you have a particular interest in the history of mining I would take the lower route from Keld to Reeth along Swaledale as it is beautiful.
If you take the valley route here then you won't encounter any real exposure on the route unless you take some of the high level alternatives in the Lake District. If the idea of exposure bothers you I would still recommend doing the high level alternative between Borrowdale and Grasmere (along to Helm Crag) - it doesn't add any ascent you just stay higher for longer which is well worth it in the Lake District. Weather permitting of course.
You will meet lots of people in September, often bumping into them each night in the pub.
You will have a great time.
tonyk is right about Ennerdale. Much easier walking along the north of the Lake - well made forest tracks - still a great walk.