Hi,
It sounds like you have everything covered. You will be exhausted from driving. You might want to look into flying into Deer Lake and leaving from St. John's. I am not sure how this would affect you cost wise I think sometimes car rental places charge extra for not dropping off the car at the place of origin.
If you like hiking you will love Gros Morne. There are a few good hiking books.
I don't know if I'd stay in Hearts Delight for 3 nights maybe you could go to Brigus for a night or two. It's really beautiful.
MapArt but out a new updated road map a couple of years ago. I think their website is www.mapart.com.
RandMcNally also puts out road maps. The one you got from the tourist should be fairly decent though and I'm not sure I would buy another. Our tourism bureau tries to keep on top of these things.
Renting a car here in August is next to impossible so I'd book it as early as you can. July would be too late.
There a number of rental places, one called Enterprise sometimes has good deals.
If you have any more questions let me know.

I think that this is a good itinerary. There's lots of driving involved, but you are staying long enough in each location to break it up nicely.

If you plan to stay in B&B's, which is a great way to travel in Newfoundland, I would advise you to make your bookings as early as you can. They do fill up quickly in August.

While you are in Gros Morne, it would be a shame to miss the northern peninsula. Unless you really want to stay in Heart's Delight for 3 nights, I would take the time to travel up to L'Anse aux Meadows to visit the Viking site. It's worth the trip. While there, try to stay at the Tickle Inn at Cape Onion. I assure you that it will be one of the highlights of your trip. More than any other place, you need to book very early, in some cases, a year in advance.

While in Trinity, try to be there for the Trinity Pageant, produced by the Rising Tide Theatre. (They also do summer theatre productions). I would also recommend a whale expedition with Dr. Peter Beamish of Ocean Contact.

I hope you enjoy your trip.

YOur plan looks really good. The only thing I'd like to add is that when you return to St.John's on the 22nd, head south down Highway 10 for a few days and enjoy some of the Avalon Peninsula.
Be sure to see Cape Spear.
Also, if you enjoy hiking, experience some of the East Coast Trail which runs along the coast on the Avalon Peninsula. We enjoyed our hikes in this area. Although not as dramatic as some of the hikes in Gros Morne, it offered great view of the ocean from the trail.

Have a great time.