Yes you can put water bottle on down tube, there are screws to mount the cage. However, if you do that you will not be able to leverage above the bottom bracket gel container. The cage would block it. Frankly if you use integrated bladder (650 ml) and two behind the seat cages, you don't need the forth on the down tube...
As for refill on the fly, Flap the cover and the opening is big enough so you can squeeze the bottle to fill it up without spilling around. Canyon Speedmax CFR has a valve instead but probably you would need to be more precise to discharge the bottle into the bladder.

my two cents :)