But the character page says niu6 is the reading when it means urine/to urinate.
Apart from our character page, niu6 for “urine/urinate” is also supported by 廣州話正音字典 (p.118), 廣州音字典 (p.107), LSHK’s 粵語拼音字表 (p.116), and all 4 sources on CUHK site. It is very unlikely that nik1 would be suggested by any dictionary when used in 便溺.
Is it true that bin6nik1 is the most common reading,
Neither bin6 nik1/lik1 nor bin6 niu6 are common readings because this term is rarely used in verbal speech. So when this term suddenly comes up, e.g. in news broadcast, the speaker would not look up any dictionary and merely pronounce it in the only reading that he/she knows. And nik1 is indeed the more common reading which, unfortunately, does not cover the “urine” meaning.
Apart from our character page, niu6 for “urine/urinate” is also supported by 廣州話正音字典 (p.118), 廣州音字典 (p.107), LSHK’s 粵語拼音字表 (p.116), and all 4 sources on CUHK site. It is very unlikely that nik1 would be suggested by any dictionary when used in 便溺.
Is it true that bin6nik1 is the most common reading,
Neither bin6 nik1/lik1 nor bin6 niu6 are common readings because this term is rarely used in verbal speech. So when this term suddenly comes up, e.g. in news broadcast, the speaker would not look up any dictionary and merely pronounce it in the only reading that he/she knows. And nik1 is indeed the more common reading which, unfortunately, does not cover the “urine” meaning.