Other Non Ikelite strobe will only work in Manual mode even when using special ikelite to Inon/Sea and Sea strobe sync cord.
That is not true any strobe with DS-TTL works with any canon camera with the flash in auto mode including old models, canon has ETTL on their cameras since the 90s. Inon, Sea & Sea and other brand will all work fine with a fiber optic cable and provide DS-TTL or wireless TTL
I keen to know about the speed override. If I am not mistaken when using flash the highest shutter speed is 1/200 sec. I wonder if the CHDK can fix this, but will it work with slave strobe?
I don't know about the Ikelite and the G12 hotshoe, typically electrical TTL system based on sync cord can sync at maximum of 1/250. CHDK has nothing to do with this limitation is the electrical signal that only goes that fast
Systems with DS-TTL do not have any limitation they sync at the fastest speed of the internal flash which is 1/2000.
I am still using the canon housing with the Sea and Sea D1 strobe, sometime I also use the fiber optik with the ikelite housing. Sometime to freeze the moment I need shutter speed faster than 1/200.
You are in an ideal position to test DS-TTL set the camera flash to auto and the D1 to TTL and you will see you can sync at any speed. Then you can form yourself an opinion.
Just to be clear the flash output always lasts less than 1/250 actually is more something around 1/5000 or 1/10000 so allowing for a faster shutter speed only changes the background exposure not the foreground
Which means you are shooting CFWA you are between 1/125 and 1/500 otherwise the background is too dark and you don't have nice blue water unless you have a lot of ambient light
The speeds of 1/500 and more are really for macro when you want to isolate the subject from the background. If you shoot in Manual the flash will shoot in any case and is not difficult to control exposure of foreground with your strobe.
So in summary if in CHDK you shoot in aperture priority with flash in auto mode and override the shutter speed say to 1/500 the YS-DS1 will provide DS-TTL out of the box you already have all the equipment you need