Konfedercja also takes voters from the other parties.
Barely. The polls are showing that Konfederacja are gaining at the expense of PiS, not other parties. They might pick up some voters from what remains of K'15, but then anyone jumping would have already jumped. The idea of PO voters moving away from PO to Konfederacja is amusing though - if anything, they're going to the PSL or Lewica.
I don't understand why other candidates don't frill him more about it.
I think it's political strategy. If someone abandons PiS for Konfederacja, they're by no means certain to vote for Duda in the 2nd round. So, better to leave him alone and let him chip away at Duda.
Bosak is doing well by sticking to economic/real life issues and downplaying the social issues, but that only lasts as long as there's a form of detente between him and other candidates. As soon as he poses a threat to the duopoly, he'll be crushed.