As you have mentioned, female nudity is common in modelling, TV and film, not for someone who wants to to become a HRH , Princess , or a duchess ( whichever Prince Carl Philip's wife will be titled). Helen Mirren might be a respected person, but in the end, she is an actress . And as for her charity event in South Africa, I can't imagine what was she thinking!! Is the honour of doing charity so much that you can even do such thing to this young girls in a country with a high rape crime?!!
To start, I do agree with your points regarding the differences between models/actresses and royals. However, I think it's equally important to remember that Sofia stopped modeling before her relationship with Carl Philip was confirmed (or at least in so far as I can tell, but I will stand corrected if someone has information saying otherwise). This means that while she did previously have nude photos of her taken for a living, she ceased to do so when she became a prospective Princess.
I cited Helen Mirren as an example because she's someone who is respected, even though she's been nude or partially nude as the result of her career. The point is that we can respect actresses who have posed partially or fully nude at various points in their careers, so I don't understand why we can't respect a woman who once posed nude as a part of her career but no longer does so because her life has taken a different path.
Yes, Sofia's got a controversial past, but she's far from being the only royal bride (or prospective royal bride) to have skeletons on display. I mean, look at some of the other royals out there. In Monaco the monarch has two confirmed illegitimate children, in Belgium the recently abdicated monarch is going through a scandal right now related to his (alleged) illegitimate child, in Denmark the monarch's younger son is a man who is on his second marriage, in Luxembourg one of the monarch's sons had a child out of wedlock shortly before his 20th birthday, in the Netherlands the Queen's family had to be investigated for her father's role in the Dirty War, in Norway the CP is married to a woman who has a child from a prior relationship, the CP of Spain is also married to a divorced woman (not to mention everything else going on in Spain), and Britain... well just to start, 3 of the monarch's 4 children have been divorced, one of whom later married his very controversial mistress and another whom is frequently suspected of carrying on a relationship with his ex-wife.
With all of that in mind, is a prospective Princess, who is dating a man who isn't in the direct line of succession, with a past in nude modeling really all that bad?
As for the Africa pictures, I don't feel I know enough about the charity, Sofia's involvement, the context of the pictures themselves or the wardrobe - not to mention who chose the tops and for what reason they were chosen - to really form an opinion on them. That said, I do hesitate to condemn them simply because it's a tricky path between "dressing young girls in skimpy tops is inappropriate because they might get raped" and "young girls dressed in skimpy tops are asking to be raped." I do agree that the wardrobe of a young girl should be age appropriate and that the pictures don't seem age appropriate - however without more of a context... I mean, while I'm not a fan of it, I would be lying if I said that I never saw girls of that age range wearing similar outfits at certain points of the year, and I don't believe that they (or their parents) are doing so with rape in mind.
This is sick to see these young girls like this, OMG, what is she thinking? In this country the female population is not SAFE from rape and horror done on a daily basis and she does this.......I really was going to give her the benefit of the doubt about her past, now no way....she is 
