Is it necessary to perform ghusl and repeat the 'Asr prayer if I felt ejaculation and prevented it, then found a drop with an odor on my underwear and doubted whether it was maniyy, madhiy, or something else, and I prayed 'Asr with only wudu'?

If you doubt whether what came out is maniyy (seminal fluid) or madhiy (pre-seminal fluid), then you have a choice: if you wish, you may consider it maniyy, in which case you must perform ghusl (ritual bath) and repeat the prayers performed after its emission and before ghusl. And if you wish, you may consider it madhiy, in which case your prayer would be valid if you washed what it soiled, performed wudu' (ablution), and prayed with a clean garment. This is the view of the Shafi'i school. As for what you did, it is forbidden masturbation.