Catch a 500 fpm downdraft out of nowhere on short final
Flaps are not a flight control, they are a lift and drag device. For the occasions when you encounter rapidly settling or rising air, you fly the plane through it. You manage pitch and power to maintain an acceptable approach path, or go around.
Your well planned approach should have built in allowance for variations, because your judgement has enabled you to plan well. In the case of a sudden and unforeseen change in the air movement, electric flaps will be much too slow to have any beneficial affect as a "flight control". Manual flaps are rapid enough, but the danger in accidentally going from full flap to zero flap at a slower approach speed far outweighs the benefit in changing approach path.
Happily, I have reviewed the flight manual prior to flying the aircraft, and know that no aircraft I have ever flown includes reducing flap extension during a continued approach as an emergency or normal procedure.
During certification testing, it will have been demonstrated that the most rapid selection from full to zero flap which can be made, can be flown through and recovered, but there will usually be quite a lot of altitude loss.
We aspire to a stabilized approach, which progressively configures the aircraft for landing. Retracting flaps would not conform to this. It "de-configuers" the aircraft, and introduces the risk that the landing gear might be mistakenly retracted. If a pilot I were training did this, we would have a long talk about it, and they would not do it again, if they wanted my signoff.
If you feel the need to reduce flap extension during final approach, you should go around, things are too far gone to continue a normal approach.
adding trim reduces the effectiveness of the elevator (by the area of the tab)
Not in any certified aircraft it doesn't. In some types, adverse elevator trim can be effectively used to increase elevator effectiveness, albeit with a lot of muscle needed.