Wow! I'm surprised someone bothered to look into this issue further. You're absolutely right about the Nestorians and Manicheans. These now-dead branches of Christianity did have an impact in China, but just as you said, to what extent we aren't able to know. The mere presence of Christianity in China must have influenced Chinese thought and religion, and therefore Pure Land Buddhism also. But in the two centuries of these religions existing in China, it's likely that their fellowship did not extend much outside the original immigrant communities, not enough to profoundly influence Buddhism. Basically, yes, Christianity must have had some influence on Buddhism simply because it had some presence in some parts of Chinese history. But there is really no substantial evidence (that I know of) that links Christianity in China to Buddhism in China.
To paraphrase my religion professor: "The only reason anyone ever talks about Christianity in China is because we come from a Christian background. If you were to see how the Chinese write their own religious history, Christianity would just be a footnote."
Finally, regarding the dates and origins of Pure Land Buddhism. It has kind of dual origins in both China and India. Two of the core texts of Pure Land Buddhism (The Larger and Smaller Pure Land Sutras), have sanskrit predecessors, which were written between 252 and 713 ad according to this source
. But Pure Land Buddhism was never an independent school of Buddhism in India, as it was in China.
Yeah, this thread has really taken a detour.
Hope that helps.