As a marker in singles, I was trained to immediately walk in front of the head so that I can properly see if there is a toucher, etc, and to avoid getting hit by flying bowls, etc.
As for pennants, I don't think that there should be a problem with the thirds doing the same thing. If the other team members are close by, they can try to protect other rinks if they wish, however, they should avoid risking being hurt.
On the point of walking up the rink after the third has delivered his last bowl, he quite rightly has possession of the rink and is entitled to be able to view his bowl as it approaches the head. Most players, including myself still tend to walk up while the bowl is in motion, but when I do it, I make the effort to walk out of his line of view of his bowl.