Must someone who used to abandon prayer and then became a Hanbali make up the missed prayers after repenting, given that before becoming a Hanbali he followed the view of those who do not hold that making them up is obligatory? And what is the Malikis' evidence that someone who prays two days' worth of prayers is not negligent?

There is no harm in being a Hanbali, and you are not required to adhere to the Hanbali madhhab (definition) in every matter. Rather, it is permissible for you to follow other madhahib in individual issues, relying on scholars whom you trust, as long as you do not intend to seek out dispensations. There is no harm upon you in adopting the Maliki opinion in the aforementioned issue, for seeking dispensations due to necessity is something many scholars permit, and this is not considered blameworthy conflation.