So, I asnwered this question which was linked to this one.

AS I said, the sentence used with noun instead of adjective are negative sentence used to express how someone is and only is (something like the main characteristic of someone, which overcome all others he can display because it is too strong).

In your context “n’être que douceur” with someone mean that you are only soft/kind with her and never have another behaviour. With the conditional (si), it mean that you have other behaviour. Without much detail, it could be that you are never kind to her, only sometimes or maybe you’re really soft with her but you know to be strict when needed.
But you said “you give her no quarter“. So the sentence now clearly mean that you make her work a lot. The other part means that if you weren’t making her work a lot and if you were only soft, she wouldn’t try to catch up with you.

On the other hand, to be “tout sucre, tout miel” is a common idiom, meaning as you say “se donner une apparence de douceur“. With the previous sentence, from my point of view it can’t be understand like this. This idiom is mostly used to describe someone as it has a pejorative connotation.

Someone wouldn’t talk about himself this way. So the sentence being interpreted this way when you talk about you (or a group you include yourself in) is unlikely.

Moreover, from my point of view, the 2 sentence doesn’t work together: why giving the appearance of being kind change from being kind (ie being only softness). Being all sweetness and light or treating her with kid gloves doesn’t make any difference in the way she see it, you would be kind to her and that’s all. And being really kind, or acting without really thinking it isn’t relevant to the subject.

Of course the rest of the conversation, the tone used, and the general ambience are others factors that can influence the understanding, but still