We all know that Blade Runner has got some huge holes in it's plot - some scenes were cut because of budget and some scenes were edited out of sequence to 'cover up' the missing scenes. Also, there are big differences between the book and the film - the movie is loosely based on DADOES.
To get bogged down in the detail of why things are the way they are just detracts from the main issues explored in the film. We're all guilty of this at times.
Don't take this the wrong way, but your questions are really only going to test how well I or anyone else, can up with a rational explanation of the issues you raise - in other words, there is no true answer, only theories offered by others.
But I'll have a go anyway :
1. Why is it so difficult to identify replicants? We know that Tyrell has photographs of all its models, and a person matching the photo is probably a replicant. It would be much easier to simply apprehend a person matching such a photo and then perform a lab-test of their bone-marrow or DNA to confirm that they?re a replicant.
Firstly, the fact that Replicants are returning to Earth is not known by the general population and the fact that the Replicants killed their masters has also been kept quiet. Replicants are being given away as an incentive to go off-world - nobody would take the incentive if they thought it might one day revolt and murder them.
I've also seen the theory that Bryant is somehow in Tyrell's pocket. Discretion in these matters is good for Tyrell's business which in turn could be good for Bryant. Also from this theory comes the idea that Deckard is a Replicant created purely for the purpose of tracking the 'rogue' Replicants. Perhaps the whole situation has been engineered between Tyrell and Bryant. Deckard could just be another 'experiment' like Rachael.
VK tests are disguised as regular IQ tests, which the general population has to take on a regular basis to measure the affect of the background radition on their brains (remember 'chickenheads'). Remember when Leon says 'I've already had an IQ test this year'?
Deckard doesn't have any trouble identifying the Replicants - he recognises Zora, Leon and Roy straight away. He has a little trouble with Pris because she radically changed her image with the wild hair and make-up. The Nexus-6 models are new. I get the impression that these are the first bio-engineered Replicants - perhaps the previous generations of models were essentially mechanical as in DADOES.
My last point on this - perhaps the four Replicants did actually change their appearance when they arrived on Earth. Perhaps they look the same as the images in the Tyrell files only to the movie-goers so they could identify them on screen. Perhaps Deckard is actually seeing something else. This might sound a little crazy, but I would put it in the same area as when you have the German officers in a war movie all speaking perfect English to one another - in reality they wouldn't, but there's no point speaking the dialogue in German and then sub-titling it for an English audience.
2. Along a similar line, how can a replicant assume another identity as written in ?DADOES?? If the Battys, for example, assumed their form from the real (i.e. human) Battys on Mars, how did nobody notice the fact that the Battys not only looked completely different all of a sudden, but also looked exactly like the popular new Nexus-6 replicants?
I'm not sure that Replicants assume people's identity in that way - they assume an identity in order to achieve their goals but don't continue to lead the lives of those people. James Bond might assume someone's identity in order to get access to a 'base' but he wouldn't invite the victims parents round to tea and pretend everything was the same, would he?
3. How come Deckard, supposedly the BEST bladerunner in the business, has to ask Bryant how come replicants come to earth? Replicants are only illegal on earth, as infered by the opening text, so therefore bladerunners work primarily on earth, to stop trespassing replicants. It seems that any bladerunner would know the answer to this question, since the answer to this question forms the basis for their careers of bladerunners - to find and "retire" replicants who have snuck back onto Earth.
I think Deckard's question is a fair one - he asks "Why risk coming back?" and "Why target the Tyrell Corporation?". He's trying to anticipate their moves - isn't that just good detective work?
If none of my explanations fit, you could trying making up your own
