Using his sword to lift the latch, or "gin" as was originally written seems sensible but how she can open an open door is beyond me unless if the latch or bolt (another definition of gin) remains in the open position it proves she did it. The sword is just the implement.
As far as Saunders sleeping "chastely", Buchan's Ballads of the North of Scotland,
Verse 19:
Awake, awake now Clerk Sandy,
Awake, and turn ye unto me;
Ye're nae sae keen' s ye were at night,
When you and I met on the lee.

The 'sweat' she felt was, of course, his blood