Of course you can make reloads in parallel, as long as they do not access the same file at the same time, because that can cause file locks, and eventually corrupted files. So make sure that each task reads from different sources should they are running at the same time to avoid this.
For example, if you have a QVW that reads master data such as Customer, Vendor, Region, and another QVW that reads fact data such as invoices, shipping orders, then they can run in parallel. Of course, you can always store the same data in two different files, for the sake of performance so this allowing a task to read from Customers QVD twice at the same time.
In regards to the other question, when your last QVD has been realoded, you can trigger a task that depends on this one, so you don't need to specify any time for this.
Hope that helps.