I have three questions:
May stressed syllables be indicated by low tune instead of high, with
unstressed syllables having high tune and low tune respectively?

Does contrast in pitch in pitch accent languages necessarily be of
the type HL vs LH? Or may H vs LH/HL be an equally possible contrast
opposition? (H-high,L-low)

Are there any known instances of qualitative changes in vowel
inventory, in which pitch accent is a decisive factor?

Such as
[á] > [ó]
[â] > [â]