If I could just pick up on a question you raised earlier .

Have you ever seen a sentence with 'if' like above that looks incomplete(it looks there is no subordinate clause but only if cluase.)

The sentence is sourced from my dictionary.
That's often heard but I'm not sure whether the punctuation is correct. I'd probably use an ellipses (which is frequently used when we expect a pause and I certainly expect a pause after that sentence)

If I could just pick up on a question you raised earlier ...
Hi

I suppose it can stand alone provided it is supported contextually.

She was always willing to help me. If (only) I could have been a little nearer to her.

I am very much interested to know what others have to say on this.