IBIS has the advantage of working for video and still though. The digital system on the canon is for video only but it does work really well from what I've seen.
Good point - though it occurs to me that the recent versions of Photoshop have (I think) a filter to reduce blur caused by camera shake (on a single image). If done in-camera with movement detected by an accelerometer (or at least with accelerometer metadata stored in the image file) it should be possible to improve a blurred image in post. Not as good as IBIS or IS, but better than nothing.
B - When going from a 30MP sensor to 4K you have to "dump" or "bin" a huge amount of the overall sensors data. Which method you choose each has it's pluses and minuses. If it were cost effective in the hardware, it would be BEST to have the option to chose either a central contiguous crop or an interlaced/lineskipped "FF" crop, (it is still a crop to get the aspect ratio from "ff" down to the wider 4K.)
Downsampling softens the images, cropping reduces usefulness of wide angle, etc. It's not a one is better for everyone. Often though, it is one is better for you.
My assumption is that the best option would be a good quality downsampling of all pixels, as line skipping would introduce some pretty nasty artefacts. Image frames can be sharpened in post, but removal of moire is much harder (I believe).
It's a shame they can't take a 6144x3240 crop from the sensor (just a 1.16x crop vs the whole frame), and downsample by 1.5x to produce a 4096x2160 video stream. A 35mm focal length becoming 40mm in 4K mode (1.16x crop) would likely get less flack than the rather large 1.7x crop.
may I ask how does the RF lens mount on the EF-M line? or does it able to mount?
AFAIK There's currently no crossover with EF-M and the R system. I guess that might change in the future (if the registration distance means that it's physically possible to make an adaptor).
On the subject of EF to R adaptors - someone (I can't find the post right now) mentioned that it was likely we'll see cheap third party versions of the basic adapter. I think that's a good call, as it should be little more than an extension tube (with the communication pins). As often noted with cheap tubes; third party air is as good as Canon air