This is a good thread, primed by excellent questions from OP, and there have been many thoughtful responses. Here are some added - seemingly harsh - thoughts that reflect reality:
Anyone who has only a 2.4KHz (or even a 1.2KHz) IF filter is either (1) going to develop an "ear" that filters out extraneous signals, or (2) will soon consider a narrower IF filter to be well worth the cost. Those who do neither will likely abandon CW operation.
Regardless of band crowding, anyone who never takes a frequency closer than 2KHz from another station will (1) never find a frequency, or (2) will have a neighbor just 1KHz from his own frequency before completing one QSO. 1KHz spacing is the norm.
Under busy band conditions, courtesy dictates staying 500Hz from another station. That is adequate to avoid interfering with anyone who has a filter installed. For those who don't have a filter installed, please understand that is not a limitation imposed by state of the art - it is a personal choice. Refer back to my first comment & choose a solution.
Under major contest conditions, signals can be spaced 250Hz apart without causing problems for those who have 250Hz filters (or 300Hz filters and a trained ear). Problems inevitably arise, however, when stations replying to your neighbor zero beat very poorly (or not at all) before they reply. There is a special place in hell for such lids, and I invite them to go straightaway to take it.
Under competing contest conditions (aka: combat), any hole wider than 150Hz is going to be filled by a booming "CQ Test". Fact of life. By that time, non-contesters will have either moved to the WARC bands or fled to eHam & the Zed to complain.
Softening the message: The fact that so many of us have fun with CW, even under crowded band conditions, reflects the prevailing cooperative attitude that most of us share. We don't want to deliberately interfere with others and consciously adjust our procedures to avoid it. We also disabuse ourselves of any unrealistic expectation of interference-free communication. Instead, we develop some tolerance, sharpen our operating skills, and upgrade our equipment as re$ources permit.
War Story: I still remember a time long ago when my only receiver was so poor that it "listened" to the entire 40 meter novice segment all at once. It was free, so I got what I paid for. Bad as it was, that receiver was handy for developing a discriminating "ear", as well as for hearing rock-bound replies (the only kind, then) from hundreds of Hz off my own crystal frequency. Not appreciating how special
that receiver was, I soon paid for a receiver that could better discriminate between signals. That set the pattern of equipment upgrades over my next 50 years in this fine hobby . . . and I guarantee that my next transceiver will likewise be chosen to "do it better" than the one I own now.
Battle Ground, WA