Why did the Narnians feel it would be difficult to leave Tashbaan peacefully?

The Susan didn’t want to married The Prince that will make him unhappy

And the Prince has ambitions to conquer Narina.

What was the plan to get supplies on board the Narnian’s boat so they could escape ?

First , let the messages to give the Pronce a haope that Susan is weakening

Then, let The Prince know ,there a big feast wlii be hold in the boat ,so them need to make preparations for their guests

Third ,in that night everyone all go to the boat pretend to join the feast ,in fact they up sails and out oars

Where had the real Prince Corin been off to while he was missing?

He was locked up somewhere by the watch

How did Shasta escape the Narnians so he could rejoin his friends?

As the real Prince told him ,he went out by the way that Prince came in .