The title of your post says “BCCH Interference”. If I analyze your first question in this regard then:
Question: What if two BCCH sectors having same BCCH Frequencies are facing each other?
Answer: As Pix mentioned, call setup failures, call drops, HO failures, interference etc. HSN does not matter because BCCH TRX has a single frequency; the BCCH Frequency (It is not on hopping)
Question: What is the reason a mobile terminal remains camped on on a cell (serving cell) even when neighbouring cells show better levels?
Answer (considering only level handovers): According to cell reselection criteria, HO takes place when a mobile terminal sees that a cell has levels that are greater than levels of the serving cell by some threshold; i.e.,
Level (new cell) > Level (serving cell) + Threshold