Will using baby powder prevent excessive sweating?

No. Talcum powder will absorb sweat that has already been excreted, but it cannot prevent sweating; your sweat glands don't know whether you are applying powder or not. Several preventative treatments, both topical and oral, are available, but most require a prescription.
No. Powder absorbs sweat, but does not change how much you make. Antiperspirants--try ones labeled "clinical strength"--should help. There are perscription antiperspirants and then other things that can be done by a dermatologist(ionophoresis and botox).