Hi Vertika,
1) To edit the channel after availability plan is created, refer the below link.
Editing the SAP PI channel availability planning - Trick
For the 2nd and 3rd issue, its because you might have not selected any check box in the day options while creating the availability schedule.
To provide availability for Wednesday, Thursday and Friday from 5-7pm one channel is sufficient.
To make the channel available for morning 1 hour and evening 1 hour you need to create 2 channels.
If you want the Wednesday Thursday and Friday availability also in the same channel means you need to create one more channel.
Regards.