I don't know about the first part, but the second part sounds like you have two samples on the same pad, set to velocity trigger.
Go to the Studio page in Spark, and you'll see the sampler there. It has 6 layers. Check all the layers to make sure only layer 1 has a sample in it.
And no, sadly, you can't drag files to the sampler. Also no, sadly, you can't adjust how velocity triggers the sample layers. It simply auto-assigns velocities based on how many of the layers have audio in them.
Hope that helps.