τὸ "πάντες ὅσοι ἦλθον πρὸ ἐμοῦ" σημαίνει "πάντες οἳ ἦλθον πρὸ ἐμοῦ," "πάντες οἱ ἐλθόντες πρὸ ἐμοῦ," "πάντες οἱ πρόδομοί μου."
These three sentences all mean the same thing? Are there any shades of meaning or nuances? or do they all remain semantically the same?
χαῖρε φίλε! ἡδέως λέγοιμί σοι ἢ Ἑλληνιστὶ ἢ Βρεττανικῇ!
Yes, I think they mean BASICALLY the same thing. The shades of meaning are too nuanced to be captured objectively and systematically. It's possible that the ὅσοι adds a bit of indefiniteness-- "all who came or might have come," or emphasis-- "ALL who came," or durativeness-- "all who ever came at any time." But these are all guesses, all attempts to read the mind of an author who is not hear to laugh and tell us that we are reading things into his words that he never meant. It's just as likely, in my humble opinion as a semantic minimalist, that John felt the need, subconsciously, for a few extra syllables to balance out his sentence.
There are three ways to answer the question "What does πάντες ὅσοι ἦλθον πρὸ ἐμοῦ mean?" 1. You can translate it into English. 2. You can use meta-language jargon. (relative correlative pronoun with exclamatory force, subject of ἦλθον) 3. You can paraphrase it in Ancient Greek. I have come to prefer the third method as the least of three evils.