What would you do if after selecting conf 0 you notice negative speed trend which is going to hit increasing VLS.
When things do not go as expected, take over.
In the scenario you describe and I can pretty much visualize it, it's a tough call to give a generic answer. Sour grapes to some, the overall best is to keep the AP engaged and see how HAL manages.
While keeping your instrument scan sharp and alerting the colleague to the situation, and placing your hand on the disconnect button. MCT for sure possible, TOGA - eh, please be mindful of SRS (go-around mode FD) coming if the slats have not fully retracted yet.
The speed trend arrow stretching into the yellow band itself is not a problem. It shows the expected result if the momentary deceleration would continue for 10 (? - check FCOM) seconds. However, by second #3 the AP would be doing something about it.
What I am getting at is (in19 out of 20 cases) that "the Something AP will do" will be more precise and accurate than whatever manual intervention you can realistically execute. The grim reality being, that for a simple speed drop / lazy pitch control moment, the self-rectification done by the AP is not only effortless for the pilot (hell it is) but also the most accurate.
Welcome to the reality, transport flying is being done by our eyes and minds, not the hands anymore.
How many hours (total experience on the bus) do you have? If not too many perhaps there's one thing worth watching the next time, worth pointing out.