The check for private listing is I think when the seller places the aircraft for sale. Then, the price is always good (I am assuming). So I guess this does not change.
When buying a publically listed aircraft, the check is when the buyer clicks buy (lease). The system checks the alliance, and applies the limit. So under the changed proposed by OP, this check for the alliance would only apply first 2 RL days (or 2 game months).
Does the moving price limit apply to alliance prices? I forgot how that works.
The absolute minimum will drop but the alliance minimum does not on a privately listed plane. Therefore, if the airline it is listed to is in the alliance, the minimum price stays. However, if it is privately listed to a non-alliance airline, then you can drop the price as the minimum drops. From what I can tell, changing the airline it is listed to, changing it from private to public (or vice versa) or the price does not change the absolute minimum price.
Why is there such a big difference anyway? All it does is penalize the alliance member. There almost no benefit to join an alliance as it is.
Money laundering prevention. Basically, over/under value aircraft sales were used within alliances to pump cash into failing airlines. While one can still bail out an airline (i.e. sell/lease back assistance) and stay within game rules, letting an airline more or less flip planes between alliance members to pump cash into them is frowned upon (and generally restricted by the min/max alliance thresholds).