I guess the "frozen account" system about banned players was made to avoid, especially at the beginning of a world, people trying to ban their neighbour to get their city (they would be the first to know it, so have an advantage).
There are many sore losers around. I know it well from a famous game site where, especially in the chess section, most best players end up someday with an exclamation mark meaning "may use a program to play" and that all players understand as "this player is a cheat, don't play with him, feel free to harass him". Happened to me too... Ridiculous since any competitive club player would easily make his way to the top there.
Undeserved bans can happen. There's no perfect system about multi-accounting.
The present ban system can be abused in the WW era... Possibly, but it would be weird to use it deliberately, and instead of achieving a fair win, having some sacrifying themselves (and high risks in the process) for a morally flawed win for their alliance. I imagine the rival alliance enraging that they cannot attack this city... OTOH, if the ban comes out undeserved, or if the alliance is good enough to overcome the fault of 1, I'm OK with their win.
The counter-proposal looks good. Froze the account 1 week, in case the ban comes out undeserved, some have time only on weekend so they'll get a chance; and at the beginning of a world, it will let other players time to notice the good target. Then the city becomes attackable, just like if the player had stopped the game. And let the best win. Then it would be unworthy for an alliance making a whole WW island banned. 20 WW cities vulnerable once the week has passed...
Should the alliance still be able to feed the WW? Yes, just think about an alliance that loses the race because they were unable to feed it during a ban that finally turns undue. Very unfair.
So I'd go with the counter-proposal. Has anyone put it in the Ideas section?