Japt, 35 bytes

Port of Arnauld's JS solution.

r"%)%d?.|%(%w\{0,3}."Ȥ?XÅn36 d:XÌu

Try it

Japt, 35 bytes

Port of Arnauld's JS solution.

r"%)%d?.|%(%w\{0,3}."Ȥ?XÅn36 d:XÌu

Try it (includes all test cases)

r"..."Ȥ?XÅn36 d:XÌu :Implicit input of string
r :Replace
 "..." : RegEx /\)\d?.|\(\w{0,3}./g
 È : Pass each match, X, through the following fuction
 ¤ : Slice off the first 2 characters
 ? : If truthy (non-empty string)
 XÅ : Slice off the first character
 n36 : Convert from base 36
 d : Get character at that codepoint
 : : Else
 XÌ : Last character
 u : Uppercase