In response to your question #1, maybe. Need to know above info as to your court procedure but generally if they haven't met the burden of a proper complaint, then you can MTD.
You file a motion to strike the bill of sale if it does not sufficiently meet your rules of evidence. Since they say themselves they don't have the complete chain of custody then they cannot prove they own the account or have the right to collect upon it.
I would file a motion to compel the answers to your discovery if you feel they didn't answer sufficiently. Regardless of what I think, it's whether the judge in your case feels they have not which will get them compelled.