What is the ruling on a gleam seen at the tip of the glans, precisely at the urethral opening, that is thought to be urine, accompanied by a strong urge to urinate, with no trace of it on the underwear, and followed by urination, istibra' (cleansing after urination), and wudu' (ablution)?

If you have verified that some urine has exited and found its trace on the male organ, and you have washed it and performed ablution, then what you have done is correct. As long as you have not seen any trace of urine or anything else on your garment and you prayed in it, then there is nothing against you, because the default is that it is not affected until that is verified.