Obviously, Ella and Tuck themselves are essential. Ryder should also be included as he is the leader of the Paw Patrol. Skye could be added as she can fly and help Ella and Tuck in case they need to reach a high place or get an aerial view of a situation. Rocky would be great too because he's good at fixing things and might come in handy if Ella and Tuck encounter any broken machinery or structures. Another good character could be Zuma, especially if there's a water - related problem in the fanfiction.
One possible plot could be that Ella and Tuck discover a hidden treasure map in Adventure Bay. They decide to follow the map without telling the rest of the Paw Patrol at first. Along the way, they face various challenges like crossing a rickety bridge and avoiding some tricky traps set by a mischievous seagull. In the end, they find the treasure which turns out to be a magical key that can unlock a secret part of the bay filled with beautiful, glowing plants. They then share their discovery with the whole Paw Patrol.
I'm not certain. It depends on the information provided on the Wikipedia entry for that particular work. Sometimes it's clearly stated, but other times it might not be so straightforward.
Definitely not. 'Tuck Everlasting' is a work of imagination. It was created by the author to tell an engaging tale rather than being based on real events.