"If Deckard was an android, why was he wandering around unchecked while Holden was busy trying to round up the Nexus 6s (before getting blasted?)"
The 'authorities' didn't know about Rachel either, the implication was that the Tyrell Corporation was producing unregistered replicants which were freely roaming around on Earth. Even if we do assume that the authorities knew, what reason would they have to object to sending one disposable machine to kill others if it avoided risking the life of a human officer.
Don't forget Nexus 6 models were psychopaths, not serial killers. The majority weren't violent* they just had the capacity for it and the greatest danger actually came from them knowing that they weren't human and were destined to die 'young'. Given those facts, having replicants who didn't know they were replicants on earth might have been considered an acceptable risk, especially since so many successful humans are also psychopaths.
All of this is predicated on the belief that Deckard is a replicant, yes Scott believes he was, but if he wanted everyone to know this the ending could have been more definitive. The reason the film (director's cut) ends the way it does is because we're supposed to be unsure, the whole film is about the nature of humanity - we're all supposed to have doubts and ultimately wonder whether it matters what we are but instead who we are. This was the core theme of the book too, although it ended differently. This is also a recurring theme in and a lot of Dick's work. He preferred his readers to think than to spoon feed them resolutions, at least to me he was much more of a philosopher than a writer.
* If all Nexus 6 models were violent then they wouldn't serve the purposes they were created for - who wants a sex worker that will kill every John or a maintenance tech who sabotages everything?