i had the exact same problem when my BB was under 3 month. i took her to see a lot of doctors, and concluded that this is due to her immature nasal development. This "problem" lasted for about 3 months. In the meantime, i did the followings to relief her:
(1) tilt her mattress a little bit so that she doesn't need to lie flat which causes the fluid to flow be into her nasal.
(2) let her sleep on her side
(4) Before she goes to bed, use a nose pump to pump the fluid / 鼻屎 out before she goes to bed (or before she had her last meal). We did this almost every night in the beginning, then we reduced the use as she improved.
(5) use the 生理鹽水 only to dissolve hardened 鼻屎.. i tried not to rely on this even though everyone says it's safe. I wanted baby to "fight" this problem more naturally.
(6) when she has sign of 鼻塞 in the middle of the night, I usually carried her so that she can lie at 45 degrees. If this doesn't work, pump her nose again.... (this may be quite tiring in the beginning, but this is only temporary!!)
good luck, and I hope your BB's 鼻塞 will be gone real soon.