I think this is already on our enhancement list, but we'll look into the feasibility/timing of this. What do you want to do where multi-select would be helpful? Adjusting Show/Hide times?
Check out this post regarding a similar topic - https://portal.smartbuilder.com/web/smartbuilder/forum/-/message_boards/message/394575#_19_message_394515
Also, when you add objects to the Timeline, it will add it at the playhead - https://www.screencast.com/t/JgfkPROEDF0r
(not great for making adjustments).