- I really could have done it, if you had come earlier.
- I could really have done it, if you had come earlier.
- I could have really done it, if you had come earlier.
- I could have done it really, if you had come earlier.
Q1. I wonder if there is or are any wrong sentence(s) here either grammatically or unnaturally.
Q2. ‘could have’ usually can be contracted like "could’ve", if so, adverbs like ‘really’ can not go in there.
Q3. Is the expression "could really have done it" wrong?
The fact that could have is contracted to "could’ve" makes me tend to believe it’s wrong.
Of your four options, the first one sounds most natural, the second isn’t bad (but is a split infinitive), the third sounds awkward, and the fourth seems to need another comma before "really." Let’s examine why. The inclusion of the word "really" is rather unnecessary, unless the speaker believes the listener has doubts about the truth of what they are saying. I imagine the conversation this way: "I could have done it." "No, you couldn’t." "I really could have done it, if you had come earlier." So, you should put "really" before the phrase that needs the emphasis "could have done it" – rather than in the middle of that phrase or after it. If you use the fourth option – "I could have done it, really, if you had come earlier" – it doesn’t sound convincing, like you don’t even believe yourself, because the "really" is sort of an afterthought.