Hmm, some difficult hypotheses there. The law is vague to say the least. I believe the confusion lies in differing opinions of what constitutes 'indecency'.
In England, Wales and Northern Ireland, it is an offence for someone, male or female, to expose their genitals if they intended another person to see them and to be caused alarm or distress (Sexual Offences Act 2003). There is also the common law offence of outraging public decency. This could mean female breasts, depending on whether a person is offended. In Scotland, indecent exposure is covered by common law.
Women's breasts are viewed presumptively as sexual and accordingly as dirty and taboo - even public breastfeeding offends some people. During the Victorian era, exposure of a woman's legs was considered indecent in much of the Western world. An adult woman exposing her navel was also considered indecent in the West as late as the 1960s and 1970s.
This doesn't answer any of your questions, but I think that the law is so vague that it is open to speculation as could be deemed rather archaic.