I think RDI sensationalizes wherever possible, and doesn't even look at the most basic reasons. If there was material found, then it could be by assault or by secondary transfer.
You asked. If you want to get into transfer, that's fine.
The most basic but less sensational reason for breaking both ends off beforehand is to have a garrote that could not be grabbed by the victim, and could be used to assault or threaten the victim. The shards in the paint tote support the idea that both ends were broken off before the assault on JBR, since it is believed the paintbrush originated in that paint tote.
I don't see how the shards say before or after one way or the other. And your idea doesn't make much sense. One, she couldn't have grabbed it anyway, since she'd been knocked into next week. Two, they supposedly had a stun gun. Three, they had Burke's knife. What more did they need?
JBR threatened with a broken paintbrush end doesn't occur in RDI theories.
For good reason, I might add.
Only sensasional claims occur.
As UKGuy was so good to point out, IDI claims are even more outlandish.
RDI couldn't care less if the paintbrush is broken at one end, both ends, or neither end.
Pretty much.
IDI, on the other hand, can easily consider that the paintbrush was broken at both ends to threaten JBR and prevent her from grabbing the paintbrush.