All I can tell you is that when I make oleander soup from plants that I KNOW have not had chemical insecticides it does NOT numb my lips or tongue- and I make quite the potent "soup". There is a very slight bit of stinging or tingling sensation after awhile when I touch the "soup" on my lips and the same on my tongue and the roof of my mouth, but it is not actual numbness as neither my lips nor my tongue are in any way desensitised to normal touch.
Are you sure that what you are calling numbness is not actually just lingering bitterness? If you place some of your "soup" on your lips, do they become numb or do they at most just have a slight bitterness and perhaps some stinging or tingling that does not actually desensitive your lips to touch?
I guess there might be a fine line between stinging or tingling and actual numbness, but I can tell the difference. I have had contaminated soup and it definitely causes some numbness in the sense of impaired feeling. I notice a similar feeling at times when I have eaten unwashed grapes and other produce originating outside the US where pesticide use is common.
If you decide that it is only bitterness (and oleander "soup" does taste pretty nasty), the best way to avoid that is to take half your dose of "soup" sublingually (under the tongue) where there are no taste sensors and mix the other half in a strong juice such as dark grape juice or pomegranate. After a few minutes, take a gulp from the cup of juice/"soup" mixture and when you swallow the taste will not be unpleasant. Then the rest of the mix in the cup will actually taste pretty good.
If you do determine that there is actual numbness, I must advise that you discard the soup. If you have some remaining plants, you may be able to save them by thoroughly rinsing the soil over a period of a couple of weeks, cutting back the old growth and regrowing new growth. The most common pesticide used by nurseries, including so-called organic ones that may fudge, is malathion. It breaks down fairly rapidly. If you have already taken some of the "soup" you have and decide that it has been exposed to pesticides, though it is never a good idea to ingest pesticides, the good news is that Malathion is eliminated from the body fairly quickly.
All the best,
Just to be sure, I put some of my latest batch on my lips and tongue and I can see how one might feel like the tingling, especially on the tongue and roof of the mouth, might be numbness.
I really have no idea how long the "soup" might last in the fridge - I would not even think of making it without using a preservative such as vodka or vinegar. Actually, I use a combination of colloidal silver and vodka or vinegar so that I don't have to dilute it 50/50 with preservative.
The most accurate way to measure the potency of the extract called "oleander soup" is by using a hydrometer or very sensitive weight scale. The resultant extract should have a specific gravity, or weight, of 1.07 compared to 1.0 for water alone.