“It would be a lot easier to slip him a portkey while Harry was in his office”
Right. Because that wouldnt be suspicious at ALL. I think that despite what people say, that putting Harry’s name in the goblet generated a lot of suspicion – while that may be true, people suspected Harry, or karkaroff, or even dumbledore, but did anyone suspect moody-crouch?? whereas had Harry just disappeared stam, the first thing Dumbledore would have done would have been to question the teachers, which was the last thing crouch would have wanted, since Dumbledore was very good at detecting lies.
“he could have Imperiused anybody else to present Harry with a Portkey”
well then wouldnt that person be transported before he got the chance to hand over the portkey?