If docker and/or VMs are enabled, but not actually utilized, the drives will still spin down. But in your case, if you're not using them, then don't even enable them.
It does improve write speed. But, for many users (myself included), I only use the cache drive for applications (ie: VMs / docker apps), and all writes for media go straight to the array. For me, the write speed is good enough, and most of the writes are all automated (new downloads etc), so I don't even notice if it's slower than if I wrote directly to the cache drive.
Yes. Just enter in the appropriate info in Notification settings