I'm not sure how Slimy could have ducked the grab, you did read it was aimed at whatever it ran into first, arm or head, and coming from a low angle, correct? 

The A.S.P is not reaching behind itself, the optics are where the neck is on an ordinary human and it doesn't have a head. It does not need to reach far over itself because Higan cut off Slimy's escape routes by ramming backwards. 

Also, I'm not sure how big your arms are but I don't see how you're not seeing the gap isn't really big enough. When I mime the action it's barely 15cm wide at its widest point, doubling that for the A.S.P still leaves a gap of maybe 1 and a half feet at most, which would be a struggle even for the Pater.

There's also the fact that you seem to think it would be impossible for the A.S.P to squeeze that gap, when it could simply drop its arm down like it were hugging a rugby ball. The moment the elbow began to dip it would twist the head of the Pater horrendously and crush its mid-section into the Mech's body.