I'm by no means a DHS expert, so it's possible I've accidentally overlayed some critical infrastructure or buildings that simply can't (or wouldn't) be moved, but here are two possible overlay examples. I believe I have the bulk of DCA's Carsland included in my cutout here (and the scales are set correctly).
In the first overlay, this is a "simple" drop onto the LMA area, assuming an entrance at the end of Pixar Place and the "Route 66" plowing dead center into from the Streets of New York. As you can see, a large swath of the Backlot tour is untouched.
In the second overlay, I've flipped the orientation of Carsland and shaded out a possible entrance (through the Animation Courtyard, I believe) and carved out the bulk of the Backlot Tour as a "Route 66" lane.
Will they do any or all of this? Who knows. But it appears there is definitely plenty of room for a full implementation if they decide to go this route.
Personally, I'm all for the full implementation. DHS desperately needs attractions and, quite honestly, I simply don't see a clone drawing much attendance from the original in Disneyland. While there are definitely folks specifically booking Disneyland trips to experience it, those bleeding-edgers will have satisfied themselves with trips to DL by the time the clone is open, and it's simply not a big enough draw to pull a significant number of people who otherwise wouldn't go to Disneyland. Anyone trying to argue otherwise is fooling themselves. On the other hand, there are a large segment of people who will either book trips to WDW, or spend more time and money at DHS, if this is built there.
As for DHS being a thematic mess, I think it's fairly obvious that DHS will, over time, turn into something much or akin to a "Disney's Hollywood Adventure". It's never going to be a clone of DCA, but with a bit of work it can easily be modified to closely mimic it. I'm certain that's the direction you will see the park take as it evolves over the next 10-15 years.