1. Where can I find availability of ferrys in the maritmes?
google where you want to go. There's a ferry to the Magdalens from PEI and I think one from Nova Scotia too. Ferry to Port Aux Basques (closer, more rural area) and Argentia (closer to St. Johns) from Sydney Nova Scotia. I think Port Aux Basques is more interesting, and it's a lot shorter. You could also drive up to Labrador from there.

2. Can Isle de madeleine be visited via ferry?
Yes, and it's a nice trip. 5 hour ish ferry ride if I remember correctly.

3. If I go east using the north shore of the St Lawrence, are there boats across to Gaspé city?
No idea.

4. Is Newfoundland accessible by car?
Well...it's an island. But you can bring your car on the ferry. I imagine it's very expensive. I hitchhiked and I think the walk on was $100 each way or more.

5. Which are the towns on the Gaspé that should be visited?

Newfoundland is amazing. But it's very far away and huge, a trip to itself. Cape Breton, in Nova Scotia, is definitely the most interesting part of Nova Scotia. The Magdalen Islands are amazing. PEI can be a disspointment, kind of tourist land. But nice.

My Magdalen Islands Pictures:

Pictures from a trip through Nova Scotia, Cape Breton, Newfoundland and Labrador:

All the best!