Unfortunately, when you activate a sodium chlorite solution, only about 60% of the available chlorine dioxide can be converted to free chlorine dioxide. This leaves about 40% as basic impurities. This means that when activating a sodium chlorite solution, the resultant solution has a low PH.
In order to release the chlorine dioxide from the solution, air is forced through the activated solution and then bubbled through water. This air picks up the chlorine dioxide and leaves behind all the impurities. This gives you a water solution with a neutral PH and a concentration of chlorine dioxide.
The optimum activation of sodium chlorite solutions using 10% citric acid is in a 1:1 ratio. Adding more citric acid will release a little more free chlorine dioxide from the available chlorine dioxide, but it mostly adds impurities to the mixture.
You can do everything with a 5% sodium chlorite solution that you can do with a 28% sodium chlorite solution by simply adjusting the amounts used. You may find the higher concentration an advantage if you need large amounts and are backpacking in the wilderness having to carry everything on your back. Other than that, you are better off and much safer using a lower concentration.
If you are careful to buffer the PH of the solution, you can go either way. Washing the face followed by a chlorine dioxide solution rinse is a traditional way to work with acne. If you rinse with chlorous acid, it will have some residual effect, but keep in mind that chlorine dioxide quickly breaks down in light, so I am not sure it would be of much help. Maybe just before bed...
We have had excellent results using chlorous acid solutions to combat rashes in the groin and under arm areas. Those areas are typically not exposed to light, unless you are sunbathing, so the reaction can continue as planned.
HCl is muratic acid and is available at hardware stores. It is used to wash concrete and to adjust the PH of swiming pools and hot tubs. You have to dilute it down. The target is to have HCl at a concentration that is 1.2 times the concentration of the sodium chlorite solution. When using 5% sodium chlorite you use 6% HCl.
The "short term" in regards to chlorine dioxide is minutes.
The 5% sodium chlorite solution, if kept in a cool dark place, is good for 5 - 7 years.
I use a chlorous acid solution as a general sanitizer. I usually mix up 500 ml at a time and it is still going strong when I use the last of it 4 weeks after mixing it up. I don't know how long it will last because I use it up before it looses its strength. Of course I store it in a spray bottle out of direct light and in a cool place.
In the industrial use of chlorous acid to control mold and mildew, an application lasts 2 - 4 weeks.
I am not sure where the 3 days came from, but there are a lot of uses that exceed that by quite a bit.