Please note the article says that he wanted actual information, not a made up story. There is nothing wrong with that. Whether or not it’s wrong to say he would then work to get Assange a pardon is another question, but he certainly had the right to say it in a conversation.
If you are promising presidential action on something then shouldn't you check with the President before you promise it?
Agree.
Essentially Rohrabacher went to Assange and asked if he had proof Russia didn't hack the DNC. It's difficult to prove the negative but he was also asking 'do you have any evidence someone else did it?' And if you do have evidence it was someone else than the Russians the evidence would benefit Assange and Trump.
Simply asking someone if they had evidence to prove your client innocent isn't a crime.
If Rohrabacher asked Assange to fabricate proof it wasn't the Russians that would be different.