I'm going to make an extreme hypothetical example to make sure I understand your point. From what you've said it sounds as though the bishops could say that in Mar 2011 if you deliberately and with full knowledge and free will, etc. eat meat then it's a mortal sin. However, in April it will not be but in Jun it will be again. You seem to be saying the literal "whatever" they decide so (though of course highly unlikely) in this scenario it would be true.
Yes, Ambrose, that hypothetical is rather extreme. It represents the bishops as “playing around” with their powers. This is something that simply will not happen because the Vatican’s Congregation for the Clergy will step in and put a stop to it. It is an abuse of the office. Furthermore, because it is a liturgical function reserved to Rome, local regulations deviating from the general norms governing fasting are set by conferences of bishops, not by individual bishops, and they must be ratified by the Congregation for Divine Worship and the Discipline of the Sacraments before they can take effect.
In the case of no meat on Friday, the United States Conference of Catholic Bishops (then called the National Conference of Catholic Bishops) established its particular norm once, in 1968, and has not changed it since. So it has been in effect, unchanged, for over 40 years. The norm was clarified in 2000 to make it clear that the Latin Rite’s universal norm and obligation are still in effect, but that outside of Lent there are added options, and the norm, as officially changed, is not binding under pain of sin. This applies to the United States only, according to the jurisdiction of the bishops establishing the norm. It still had to be ratified by the Vatican (which it was) before it could take effect.
The Church’s rules are not arbitrary. Bishops’ choices are limited by canon law, and many of them are subject to Vatican approval as well. The Church’s law is set up take care the majority of cases and is interpreted and enforced by the the central offices. And you are forgetting that the Church is not a purely human society, but is guided by the Holy Spirit. The faithful will not be “sold down the river” because God won’t allow it.
I'm fairly sure there are areas of disobedience to a bishop that are not "under pain of sin" -- locally for instance the bishop has decided the faithful are to stand right after the agnes dei. Many disobey because their whole lives they have knelt.
This is a delicate situation because there happens to be no Church law that specifically says an individual bishop cannot set a norm such as this for his diocese. However, the Vatican has also pronounced that the faithful cannot be bound to such a norm under pain of sin because it is contrary to the established practice for the entire Rite.
So in both of the instances you cite, there is no sin attached to disobedience. And one way or another, abuse of power in the Church will be nullified by the rules already in place or through an overruling by the pope or the Vatican’s central authority.