While it is true what others have said, that if they would stand close to each other the moment the ability/mechanic gets cast, the distance they need to travel is shorter. However that's just the explanation for dealing with that kind of mechanic. Matter of fact is this and to answer your question. No matter if they stand close or even far away from each other, the moment that chain connects 2 players, means they still have to move or it wont break the chain. Now it's up to the players to decide during the run how they want to handle it, a) as stated above, stay close to each other and you have to move to the opposite direction of the area to break it, or b) stay as far away from others, and then just move a tick to the left or right to break the chain that way. Either way, standing in one place no matter what option they choose, won't break that chain. Neither will if just the dps/tank runs away and the healer for example decided to stay in place.
To your other question, even so you can tank the boss right where he stands, especially since he returns to the middle every time anyway to cast his holy bs, whatever that move is called lol, I would suggest playing it by ear. Meaning, throughout the run as a tank you get a pretty good understanding of your groups abilities, if you saw that they couldn't dodge the boss mechanics prior from the earlier ones, or even on trash, then yes tank the last boss right at the entrance (purple line). That gives the group more time to dodge the horses and fires, however if your group has like most DF groups, those kind of people that can't be bothered to dodge any mechanic, than you're screwed regardless. Hope you can find some groups and finish leveling your tank. GL.
