Some weaves are described as having edges, particularly shielding weaves. In Moiraine's duel with Merean and Nynaeve's duel with Moghedien, both described how they kept having to resharpen the edge of the shielding weave as they battered it against the other person's connection to saidar to try to cut them off.

No, not necessarily. The "indefinitely" part is true, but the "any number of enemies" part is not. There's a limit to how much of the power the channeler can channel, which in turn limits the physical distance they could cover. A super-strength channeler like Rand might be able to channel enough air to cover a miles-wide battlefield, but a middling or weak channeler would not be able to channel as of the power to cover as wide of a space.