I may be wrong on this next point-
in both cases, the murderer made a similar effort of avoiding the navel.
On the contrary, your observation there may have more to it than the others. It's certainly true that the Ripper skirted around Eddowes' navel, because (a) Brown tells us so; and (b) we can see it for ourselves on the photographs. It's also true that "part of the belly wall, including the navel" was absent from the scene of Chapman's murder, which might suggest that her killer cut around the navel in her case also.
According to ripper author (and surgeon), Wynne Weston Davies, that is something surgeons are trained to do.
Or, presumably, something which could be read about in medical books.
To be clear, we don't actually know that the Ripper neatly skirted around Chapman's navel, only that a chunk of her belly containing the navel was missing from the scene. This isn't quite the same as "leaving the navel on a tongue of skin", which we get in Eddowes' case. Certainly, there's little else in the manner in which Chapman was dismantled that suggests that a trained hand was responsible; on the contrary, her abdomen was cut open like the crust of a meat pie.
it's not a profile in choice of victims pierre; for now, i'm settled on "prostitute" satisfying that matter.
with kate eddowes, his cuts form two flaps which he may have been able to pull back so that he could reach inside and take her uterus. she was a thin woman, so possibly that was all that was required.
with annie chapman, she may have had more belly fat that would need to be removed first, possibly cutting flaps wasn't enough or would have created difficulties when he tried to open them since there would have been more mass to contend with...
there,s nothing new, only the unexplored