You have all of the private key except 20 characters (80 bits)? Then it is possible and feasible to recover the entire key using baby-step-giant-step or pollard's tho algorithm.
It will require 2^40 work, which is a few hours on a GPU.
1) Only if you know the public key too (not the address).
Additionally if you don't have the public key, you need to generate public key/address and check each of it on blockchain explorer or full node client RPC/API which is huge overhead.