Calculation of Inorder Traversal Complexity
In order to analyse the time complexity of a tree traversal you have to think in the terms of number of nodes visited. If a tree has $n$ nodes, then each node is visited only once in inorder traversal and hence the complexity is $O(n)$.

Here, the input is in terms of number of nodes in the tree and the hence the complexity.

The notion that each recursive call is $T(n-1)$ is wrong. Let say we have a tree that has three children to the left of root and three children to the right of root. So n = 7. Now when you visit root then you have two paths. One to left which has 3 children and one to right which also has 3 children. But $7-1 \neq 3$. Hence this notion of 2 recursive calls of $T(n-1)$ is wrong. 

It totally depends on the structure of the tree. In worst case a tree may degenerate to linked list  (a skew tree having children on only one side either left or right) and we know well that traversal of linked list is $O(n)$, for a list of n elements. Hence, also the worst case is $O(n)$.