Puzzles: Wormtail's finger and Shrieking Shack tunnel

[a_reader2003]

No: HPFGUIDX 79295

I have just been re-reading PoA and two logistics problems occurred 
to me:

(1) How did Wormtail simultaneously cut off his finger and blow a 
hole in the ground whilst he was holding his wand with both hands 
behind his back ?? Note also, that Sirius says 'I cornered him', 
suggesting to me that Peter did not come there willingly to meet 
Sirius, but had come for some other purpose initially.

(2) The tunnel from the Whomping willow to the Shrieking Shack is 
initially described as so low and narrow that Harry and Hermione had 
to crouch double to get along it. Then, as Sirius and Remus talk 
about their schoolboy exploits, they explain that Sirius and James 
transformed as a bear-size dog and a stag with antlers, but were 
still able to follow Remus down the tunnel. Later, four adults, some 
tied together, manage to walk back along it, including Snape in a 
more-or-less upright position ! Hmmmmmm 

Any theories ?

CW