(1) Doesn't the spray get droplets of feces-water mix everywhere? (Supposedly ideal bathroom hygiene is to close the toilet lid before flushing, to avoid sending a mist of toilet water into the air. Any forced-water rinsing seems to guarantee a mixed-mist reaches everywhere, including parts of the buttocks and legs far from the rectum that are never contaminated by paper-wiping.)
Not really, it depends upon the spray. Most sprays are gentle, not the high-pressure hosepipe you seem to be imagining.
(2) Short of absolutely drenching yourself with multiple power rinses, how do you know you're "done" - that none of the remaining dampness is still fecal-contaminated water? (White paper provides a visual completion indicator.)
You can always check with paper. Other than that, you check with your hand. Still much cleaner than leaving dried fecal matter in your rectum.
(3) My very rough impression is that fecally-transmitted diseases (typhoid, hepatitis, cholera, anything diarrhea-causing, etc.) are more prevalent in rinse-cultures. Are you sure that "a lot of South Asian countries" should be the "ideal way to wash" model, as opposed to the other way around? (North American practice would change very fast if there were evidence paper-wiping was insufficient to protect health.)
That has more to do with the water supply being contaminated with fecal matter, and not enough fresh sources of uncontaminated water being available for drinking in those countries.