Answered by Sidi Moustafa Elqabbany
I see that part of my question on purifying a garment has been answered under the answer entitled indiscernible najasa.

To purify a garment affected with a drop of filth (be it urine, pre-ejaculatory fluid, or anything else [1]) that has no physical characteristics, simply flow plain, unused water over the garment until you’re sure water has flowed over the filthy spot. For example, if you know that the filth is on one of the sleeves but can’t remember which one, flow water over both sleeves. If you completely lose track of the filthy spot, flow water over the entire garment. The water that flows over the garment is pure (but not purifying) after separating from it.
On the other hand, soaking a filthy garment in less than qullatayn [2] of water will cause the water to become filthy and will not purify the garment.
Allah the Exalted knows best.
Notes
1. Other than filth from dogs or pigs.
2. Approximately 216 litres.
References
Indiscernible Najasa (Filth)
Moustafa Elqabbany
Metro Vancouver, Canada