1. Yes, 8 blocks in one 51 mframe. As a consequence : 8 different users can be served at the same time on the same timeslot.
2. One block equals to 4 radio bursts. It’s just how many bursts need to be used in order to transmit data contained in one radio block. It’s the same with TCH : one speech frame is sent over 4 radio timeslots (in average… not taking into account interleaving 😉 )
Same also in GPRS : 4 radio timeslots for 1 radio block…
3. Yes : plan 1 SDCCH/8 for 2 TRX in standard network.
For instance, for 4 TRX, you need 2 SDCCH/8.
More might be needed in case of high usage of SMS.
4. In case of failure of the physical unit that carries the BCCH, the whole logical TRX is transferred onto another unit. This is the reason why we must put one SDCCH/8 along with the BCCH. Other SDCCH can be put in different TRXs though.