How the process of encryption and decryption works,
In general if message is encrypted by X's private key then it can be decrypted only be X public key. Similarly
If it is encrypted by X's public key then it can be encrypted by X's private key.
Here in question sender sends message signed by S private key so any one can decrypt message using S public key.
If any 3rd party person want to send fraudulent message he has first decrypt message with S public key, do changes in message but once changes have been done it has to encrypt it with his own private key( because only S have its private key no one else)
Now even if the fraudulent message is received by R, S will be not accountable as fraudulent message does not have any signature of S on it.
Thus option II and III is not possible.
Only S himself can launch birthday attack changing content of message and later claming it was not him popular know as repudiation, which is a security violation