If we want second players to appear by the S51 draft, we have til S49 trade deadline to make this official. I think that's load of time. And it doesn't seem like anyone's really started trading S51 draft picks and in any case I don't think that matters. There wasn't 5 seasons heads up for expansion, it was announced before S30 for S31.
I think the second issue you suggest can be dealt with by expansion but I don't know if it will be an issue especially if there's a staggered start and limits on how close the two players' draft seasons can be.