Hi, ab,
Cutting to the chase, you're probably going to have to remove your window a/c. If the board decides to prohibit them, they may do so, and a judge will most likely side with the board because it has the right under New York State corporate law to run the business as it sees fit (as long as they're acting fairly).

But you think they may be singling you out, which wouldn't be fair. However, you also mention that the e-mail(s) you have received refers to pedestrians on sidewalks being hurt. That is probably because it was a standard e-mail that went to everyone. So it sounds to me as though everyone is being treated equally. If you still think you're being singled out, you can submit the apartment numbers of your neighbors who have window a/c units or simply point out that you're not the only one violating the rules.

You also suggest that the safety of your a/c is irrelevant because your windows don't face a sidewalk. But even windows that face an alley (or other service area) present a danger. After all, the laws of physics apply no matter which way a window faces: an a/c can fall even if it's over a rarely used space. Your super could be killed if an a/c falls on him, no matter what you call the spot he's standing on.

The city requires window a/c units to be properly installed with metal brackets. In my building we require a licensed professional to install them (yes, the city licenses a/c installers). You can be fined if the fire department or building inspector decides your a/c is improperly installed. This is rare, but most people have never heard of it.

As for the lack of a "posting indicating that window AC's need to be removed," posts don't matter -- although they're a nice courtesy. What matters are either the house rules (which may point out specific rules about things in windows) or your proprietary lease, which gives the corporation the authority to decide what can and cannot go in a window.

So even if window a/c's were once permitted, the board may have changed the rules.

If you're still hot under the collar, save yourself some money and check both the house rules and proprietary lease before contacting an attorney.

The bigger question to me is this: how are your wall units blocked? From the inside or the outside? If they're blocked from the outside, the co-op is responsible for clearing the obstruction; I would negotiate to keep your window units (assuming they are properly & legally installed) until the wall units are unblocked.

If they're blocked from the inside, you will have to undertake the work to clear them.

The bottom line to remember: the co-op owns the windows, and has the right to decide what goes in them. It's one of the choices you relinquished when you signed the proprietary lease.