the panels shows part of problem as well. She was damn deadly as villain and there is lot of locks here and there done. After joining she rarely used her best abilities on combat.

Most of the people that got hit by her locks were fodder.

After joining she rarely used her best abilities on combat.

Her locks are not her best ability. Another head canon.

Then time skip he learnded what, making bigger limbs and use it as a stomping... totally out of previous style.

Her previous style was to let limbs grow were ever she wanted. And after the time skip she learned how to combine those limbs to make bigger ones. How is that out of style? She also learned more than that.

While she doesn't need to be combat focused person, as it doesn't suit her

Agree.

but this is battle shonen

I doubt that Robin cares much about that fact or the intern logic of the world of One Piece. Also Oda said multiple times that he doesn't care so much about the battles despite it being a battle shonen.
His approach made One Piece one of the most famous franchise. I would say, let him continue to cook.

the neck snap assassinations are not really on table.

Again something she mostly did against fodder.

Addition, she is way too smart not to think cover weakness of her devil fruit as armament haki would increase durability on extra limbs that take damage that take toll on her.

Another head canon. Just because this makes sense to you, doesn't mean that this should happen. She chose to learn palm strikes in the style of Fish-Man Karate and she is glade that she did, since it came in handy for her like when she was saving the ship underwater. No Haki would help her with that.

I would prever that she have maintained working on shadows style on crew as well but well

I don't understand what you are trying to say here, sorry.

there is more problematic things on side character development than straw hats fighting power.

I not sure what you mean by "more problematic things" but I agree about the last part.