I think I've seen Yookyung once from a re-roll. Can't remember it is was from a re-roll I had generated or another card team member lol.
She's become a WG Yubin or perhaps even more stubborn tbh, as she's had FAR more chances to get generated on account of all the re-rolls for 3rd batch.

As of right now, no.
LOL! The last part is so true when it comes to the 1st/2nd batch cards right now. People have re-rolled so many of the "untradeable" cards from those batches that some of them have now become some of the rarer cards within their batch.
As for re-roll I get why people do it of course. If they only have interest in a few sets and can't find a trade for some of the cards they have, they'll take their chances with the re-roll.
But I'm with you on the whole "card advantage" as to why I've only re-rolled once (maybe twice, can't remember). I won't be doing it again as I like building, collecting and trading too much and having the cards I get from the ol' genny means I am slowly gaining ground on being able to get quite literally, ALL the cards LOL. I have future plans on what I want to do with the common sets I collect but don't want (like some BG sets etc).