signrawtransactionwithwallet isn't going to work on an offline wallet because it does not know any of the inputs that are being spent hence the "Input not found" part of the error.
In the second argument to signrawtransactionwithwallet did you pass the list of previous transactions for the inputs you are trying to spend? It is a JSON array so you can decode the previous transaction hex strings to get the required information to this RPC call. It has this format:
...
Arguments:
1. hexstring (string, required) The transaction hex string
2. prevtxs (json array, optional) A json array of previous dependent transaction outputs
[
{ (json object)
"txid": "hex", (string, required) The transaction id
"vout": n, (numeric, required) The output number
"scriptPubKey": "hex", (string, required) script key
"redeemScript": "hex", (string) (required for P2SH) redeem script
"witnessScript": "hex", (string) (required for P2WSH or P2SH-P2WSH) witness script
"amount": amount, (numeric or string, required) The amount spent
},
...
]