2. .I ask if Wikidata can involve a description logics layer for properties that can be later used to find data inconsistencies within Wikidata.

This exactly one of the use cases we use ShEx for, i.e. inconsistency or disagreement between resources. Wikidata does not reflect the true, but (partly) reflect what other resources describe on a selected matter. There are many examples of Wikidata where different items state opposite statements. This is fine. Any issue known on Wikidata, provided the references are correct and the values are not distorted in the ingestion process, basically needs to be discussed with the primary source, not with Wikidata. ShEx provides the instrument to identify items relevant to your view, versus items disagreeing with your view on the matter.

4. We should either change CC-0 into CC-BY Licence or try to convince the creators of ontologies to change the licence of their works into CC-0.

I am excited you are joining us in our quest for more CC0 data. More and more data is adopting the CC0 license. I really like the following blog on the matter https://osc.universityofcalifornia.edu/2016/09/cc-by-and-data-not-always-a-good-fit/

7. I think that we should be an ontology for the stages of diseases.

Wikidata is not a primary source, point me to a ontology of stages of disease and I will add it to wikidata.