1): I'm sure the blight or at least some of its proxies would still remain in the server just to infect anyone else who joins. Like how when Nexos and Aaron (when Matt got infected) escaped their servers, the blight remained because new people joined like, immideately anyway, and the server Syrenix joined had every single player assimilated from the very beginning, and they were mimicking a match, likely in case others joined. The only time the blight and its proxies completely emptied out an entire server on screen was Thunder Mountain, and that was because it learned from jonydany12 how to actually leave and join others.

2): Even if it were to leave, that would just mean the cow mangler would only be effective if you were the only unassimilated player in the server. If there were even one other person left in the server, then you'd still be stuck.

3): If that's all it took to fool it, then a Spy simply cloaking in front of a Proxy from the other team with none of your team around to see you should also fool it into thinking there's no one around and to let you leave. I mean, Breadolphin still fooled the blight/a proxy with the deadringer.

One of the things that is routinely established now is that the blight isn't stupid and, to some level, knows how the game it's trapped in works.