Secondly, it would have been fantastic if Nyreen was recruitable. Her death made no sense save to motivate Aria to go berserk and even that could easily have been done without killing her off.
How hard would it have been to record a few dozen more lines for Nyreen on the Normandy, a handful of in-mission reactions and a single exchange in London? While it would have been nice for her to interact with the others, it's not like it would have been a big deal if the budget didn't allow it. I don't think Zaeed or Kasumi ever interacted directly with anyone and I think Allers only ever speaks to Shepard or Traynor (more evidence she was an afterthought btw!)
The problem there isn't the recording, it's integrating her into the rest of the content. For Mass Effect 2
, Bioware had to include most of Zaeed and Kasumi's content on the retail disc, since all of their assets need to be baked into the various levels. I doubt Mass Effect 3
changed the engine drastically enough for that to be back on the table.