1. Ventress has no way to harm them with the force, Grievous would get overrun and Dooku probably could but his lightning is inferior to Sidious's and Vader's raw telekinetic power. So yeah Ventress dies rapidly and then Grievous and Dooku get overwhelmed and die
2. Talzin's force feats are somewhat better and her feat against Sidious is better t Han anything by Geth IMO.
3. When he finally cut loose Plagueis overwhelmed Venamis iirc, given he's vastly more powerful than Maul the Zabraks presence won't change anything
4. Don't know, don't care
5. Probably not