The summoning dolls will turn red just before they reach their limit. By boundary, the game was revering to doorways, stairs, cave entrances, etc. It was just a coincidence that the distance range was reached at the bridge.
The summoning dolls are not that tough to start with, but you can give them your old equipment (depending on the doll). If you are playing on Easy or Action, you can get better stats for the doll if you temporarily switch to Tactical before summoning them for the first time.
You can upgrade the summoning dolls' range and level, but be warned that these skills can not be unlearned. You do eventually succeed in getting separated from the DK, so it would be better to use his skill points to invest in summoning doll skills, rather than your hero's skill points (particularly if you might want to play in the after game BFs).