long Stanley blades the snap off type and plenty of lube or washing up liquid
a really sharp fillet knife might do it , but its gotta be razor sharp
, just stroke along with a bit of pressure
the 4 screws you removed went through the kit, theres probably an expanding raw-plug type fixing on the backs of the rubber sills , so cut upto these ,and around them,
or through them , they are much tougher than silicone
you could use an oscillating tool but the helper has got to be sensible
and not just a bod with no skills or patience, they can`t even make decent cuppa Tea !