1. It's by default. The block has css class ipsResponsive_hidePhone
2. I thought to add a setting. But added css rule 200px. You can edit app css file or add your properties in custom.css.
3. I think about it.
4. I thought about it. Add setting to set global image or per forum.
Inbuilt cover photos hasn't option to set link.
1. No. Ask @gerard001 https://invisionpower.com/forums/topic/432286-thread-cover/
2. Nothing if the user hasn't permissions to manage cover.
Black block if the user has permissions to manage cover.