1) During namaz zohr and asr, are we allowed to say the bismillah loudly?

2) If someone make a mistake in the zikr of tashahud, can he correct it with sajda e sahw?

3) Is the black khol an obstacle for wouzou?

1) Females can if there is no na mehram around.
2) Yes it may be done.
3) Any natural obstacle is not considered an obstacle.