I wonder if it is possible to do inbetweens on them or just full on and off...?

You mean you would like to animate the picture? Hmm, what we have according with the principle of typical E-Ink operation: basically we have two stages of the new image output. First the display controller have some kind of internal framebuffer memory. So we have to clock the picture thitherwards by SPI, everything is clear. But then follows the stage to translate the picture from memory to the glass! You put the special commad sequence and then comes...the magic. The controller puts a special voltage waveform to the glass which properly shakes and turns microscopic ink particles around its axis. Have you ever held sometime "E-book" in your hands? So you know, at this moment the screen flashes, inverts and does all sorts of glitches. And only after a split second the picture becomes to normal. Its necessary to turn correctly as many particles as possible. So what kind of animation can we talk about?
But the biggest secret is that the controller itself doesn't know the waveform which it must use for your glass. You must upload appropriate digital samples array at the initialisation stage. They call these arrays "LUT" (look-up tables). In the reference code me found two such arrays named "lut_full_update[]" and "lut_partial_update[]". When I upload the first one I've got such effects that was mentioned above. But when the second one the picture almost does not blink and draws noticeably faster! But for the price I also have some "ghosting" effect when black parts of the picture are repainting in white. Me suspect that "lut_partial_update" is some kind of reduced waveform sequence intended for adding some small areas to the main image. For more complex image variation not all particles have time to turn properly.
So looks like...it is quite possible to make some simple (and slow ) animation on this screen using this reduced waveform array but get ready for some screen garbage consequently. And keep in mind the controller's framebuffer can hold only one image at the time. Did me answer your question?

I didn't think of animation at all, but being able to do levels between black and white, so partial ink alignment. But I guess while it is possible, the controller itself sets restrictions on what you can achieve.

Mhm, so you mean grayscale or even color support... No, this one of the simplest devices of that kind so it can show only black (well, grey to be more faithful) and white pixels. But of course you can try to use "chunky" graphics like in ZX-Spectrum times.

Nice!

Do you have any idea how much is retention time of these displays?

And another question, can they be "erased" using a strong magnet or an EMP discharge?

This chinese manufacturer writes about one million guaranteed full image updates in the datasheet but in reality...who knows.
EMP you said...in consideration of typically low allowable electrostatic discharge level for such display controllers me can assume that it will die first.

Ah, good. I thought about image retention, not R/W endurance. Like, if you send an image to the display, how long can it hold it before pixels start to go back to their blank state.

Hmm...well, me saw a recommendation in the datasheet to update the image "at least once a month". But looks like it doesn't mean the picture will disappear immediately after this time. At least when me got this display it already contained the manufacturer's logo image (seems it was uploaded there during factory testing). And this image looked clear without any visible degradation. And how long the display was preserved on the warehouse between fabrication and my purchasing...right, who knows.