Is platelet count always decreased with thrombotic thrombocytopenic purpura ?

Yes, as revealed by its name: "thrombocytopenia" is Latin for low platelet count. However, the count might return to normal with effective treatment, even if (technically) TTP still is present. However, if you feel outwardly well, you can assume you don't have it: TTP is always seriously symptomatic, often dangerously so.

Yes. Visit this site for more info: https://medlineplus.gov/genetics/condition/thrombotic-thrombocytopenic-purpura/#:~:text=Thrombotic%20thrombocytopenic%20purpura%20is%20a,brain%2C%20kidneys%2C%20and%20heart.