1. The prediction error would be the fact that... well... it's not predicted at all! Nor can it be, it's done via a touch function and... apparently Touch functions are broken client side...
2. What Bekey said. Someone else also once suggested that releasing grapple button and then pushing it again could toggle the mode. Change that to be instant fire of blue instead and it'd be exactly what you want, while still allowing people to grapple launch.
Anyway, I'm currently doing experiments with the UDK, so it might be GraviNULL is moving to UDK

It's got it's ups and downs so far for me, much like Source hehe, but hopefully I can handle mines alot better in UDK, because there's also the issue of moving triggers(which the mine's are in this case) can't touch the invisible, static and non solid triggers... which is a pain in the ass since it seems to be an engine limitation.