scanning solitary reaper

I am really confused while scanning this poem.

Behold her, single in the field,
[this one is clearly iambic tetrameter]

Yon solitary Highland Lass!
[is this iambic or trochaic? 'yon' is a problem for me.]
should it be YON/ SOLItari/HIGHland/ LASS [which makes it trochaic tetrameter]
OR yon SOLI/tari HIGH/land LASS/ [which makes it iambic trimeter]

Reaping and singing by herself;
[this is confusing]

Stop here, or gently pass!
[this is also confusing]

please help
thank you