Yes, this is the same sealer that comes with the aluminum seamers for use with aluminum gutters. The tiny tubes that come with the seamers is difficult to apply. This is applied with a calking gun and there is enough to really ensure no leaks.
Probably just scrape it off
Yes; make sure your gutter is dry before you apply the sealant.
About 2 hours
This is not what the product was designed for so it would be difficult for me to say if it would perform as you need it to.
Yes, but surface needs to be dry.
Yes, it sticks to all kinds of materials including PVC. Make sure your pvc is dry before you apply this gutter sealant.
A fresh clean surface would be more ideal, however if the old sealant is not failing than sure try a top coat. Don't know why you would want to, unless the old coat was failing.