1) no the MS in dedicated mode does not listen to its BCCH. All necessary layer 3 msg are sent on the TCH (via the SACCH)
2 it depends on your TRX PREF MARK. Perhaps the calls are setup on the BCCH because there is a higher priority there.
Normally, if the calls are sent to TRX2,3 and 4 rather than 1, then you should at least see some call setup failures, because BCCH and SDCCH are interfered.
And finally, not all calls are sent to the TRXs 2/3/4. When all other TRXs are full (or if the 1800 TRXs cannot welcome this call due to poor radio conditions), then the BCCH TRX will welcome TCH as well.
Example… If 10% of calls are sent on BCCH trx, and if all of them are dropped, then there will be a drop rate of about 10% 🙂 Even more, because there will be a faster tch turnover on the first trx.