Is the spray scattered during ablution considered najas, or pure water, given the presence of residual urinary wetness after washing?

The default assumption for the splattered droplets during istinja' after urinating is that they are water, unless there is a strong presumption that they are residual urine. It is recommended to perform istinja' in a place other than where one relieves oneself to avoid obsessive thoughts (waswasah) and urine droplets, unless the place is secure, like a toilet. A Muslim should not open the door to waswasah for themselves.