Where I am sympathetic to the two fatalities described here, I don't see how either makes pacelines more dangerous to someone susceptible to head injury. I know of three folks that have had head injuries from cycling crashes (2 20 years ago, 1 more recently), and all occurred when the riders were riding solo. These injuries were made worse by the lack of company, leading to longer delays before treatment. Based upon this, the OP should not cycle at all.

Edit: as frame of reference, in the group rides I deal with, I know of five crashes with zero head injuries, but broken collar bones, broken ribs, and broken fingers.