You can queue only people who are not shielded. After you shield you can get attacked a few times by people who happened to queue your team before you put up a shield, but overall you get attacked less because you’re not visible to others.
It does both, which is half the reason people collude via Line: if everyone above you is shielded then you can only find matches with people with lower point totals, so it is beneficial to know when those other players unshield so you can target them.
If they did change this, it couldn’t be as simple as making shielded players targettable. Since they can’t lose points, the most effective strategy would be to find the highest level shielded player you could beat and repeatedly attack them. They’d probably need to either introduce a limit to the number of times you could attack a shielded player, or give diminishing returns for attacking a single player multiple times.