Is there any way to select all files in timeline at once, or few at the line at once, like your select files or folders?

Only Ctrl+Click for multi-select.

OK, so if I have 150 clips in the timeline and delete the fourth clip, how do I move all of the subsequent clips to the left to take up the empty space left after deleting the fourth clip?

If I click right in the empty space and try to Remove Space, it tells me it cannot remove space in a track with a group. I did not intend to create any groups, but I did drag all of the clips down from the Project Tree in one operation. Did that create a group?

I am simply trying to avoid doing Control+Click 140+ times in order to move all the clips to the left on the timeline.

Obviously, I am a newbie with kdenlive. I am excited about the program and look forward to becoming proficient with it.

Thanks in advance for any help.

Use the Spacer tool.

You can shift-click-drag for box-selecting clips, just as long as you start clicking at an empty space in the timeline.