(1) For pronunciation, “一” changes tone depending on (a) its meaning in the sentence or/and (b) the tone of the succeeding character (Yes I agree they are quite a handful to deal with). There are 4 scenarios:
(2) For Pinyin, it doesn’t matter whether the tone changes, always write the root tone “yī(1)”.
Example:
(1) 一心一意
Pronunciation：yì(4) xīn(1) yí(2) yì(4)
Pinyin：yī(1) xīn(1) yī(1) yì(4)
(2) 一生一次
Pronunciation：yì(4) shēng(1) yí(2) cì(4)
Pinyin：yī(1) shēng(1) yī(1) cì(4)