Thank you very much
It was sad to see most of them missing, but no worries though, I did learn during the process anyway, so I'll be better prepared in case is comes up again!
In addition to what we already discussed, I believe you are right about the environmental stress. I didn't mention about the temperature and light cycle factors. Here in Greece it has been really hot during the summer, meaning room temperature fluctuates between 27-30C (80-86F) from day to night, during the day being 29-30C (84-86F). Since these values are room temperature, they are tank temperature too (outside temps are a lot higher). The timer sets the lights for 8-2-4 hours on-off-on respectively. Maybe that was too much light for the shrimp, given that there are no really dark places to hide in this 1 gal bowl. Therefore I think that the shrimp might have been stressed due to the high temperature and lots of light, so maybe they didn't respond well to the initial treatment (gdex on its own). Perhaps that's why there were casualties while I was away too. My brother did feed and do water changes, so it wasn't totally neglected.
The single remaining shrimp (dark red male) looks totally clear now. He has no signs of infection any more, so I moved him to the 4gal tank to be with the rest rcs and the amanos. I'll watch them closely to monitor any signs of the disease occurring and act accordingly. I did a 90+% water change at the bowl and introduced new shrimp there (blue velvet/dream), which were drip acclimated with the same fresh treated tapwater that I filled the bowl with. I am now leaving the bowl light on for 8 hours, while the rest of the day it is just ambient light from the room that reaches the bowl. I am thinking of getting some dritwood, chollawood or coconut to make a hide for the shrimp just in case. Maybe that was the reason they haven't been breeding at all...
In conclusion, I believe it was the high water temperature and too much light that might have weakened the shrimp and made them more vulnerable to the disease. The salt dip (1tbsp/cup) proved effective in combination to the gdex treatment (made according to the instructions leaflet). If anything new occurs I'll post updates, but hopefully I won't have to!