The investiture controversy is the pivot around which the state-church relation swiveled for 1000 years. It is not one example among many, it is the crux of state-church relations in the Middle Ages. Prior to Gregory VII the feudal protostate was weak and power was disperesed between many lords, between which the vassal was able to choose more or less freely. Such protostate, however, was able to invest bishops, although they were never quite independent from Rome, which controlled the doctrine they taught and adjudicated. The Gregory VII revolution was a reaction to consolidation of power by emerging kings. It succeeded in providing a stronger check on the lay power of kings in the bishops now appointed from Rome. Both before and after the controversy the justice system remained dispersed between lay and church law. The very fact that the controversy took place is sufficient to prove my point, that neither the church ran the state, nor the state ran the church throughout the Middle Ages.
constantly making new law to replace inconvenient old law
The Canon law changed very little to this day. Its core is the Divine Law, which is incapable of any change at all. Local laws may have been changing, for sure, -- if you have a specific example we can discuss that. Do not forget though that we are talking about a period of roughly 1000 years.
The only actual real world difference is that today people generally have a right to look at the records and a small say on what adventurism and splendor the money is going to be spent on.
Our system provides an illusion of access and control, and the result is a stockholm-syndrom populace that is unprecedentedly docile by historical standards. 50% of our income goes to the fine-tuning the political system of the Middle East and the Balkans, shooting useless rockets into the empty space, forcing an inefficient retirement system of the working class, and perpetuating miseducation and depravity at home through the welfare system. Absolute monarchy is not my ideal, but I'd much rather have a Sun King eating big dinners at Versailles and ignoring me in my kitchen.
I would agree that what you describe here did have its occurrence to a small relative degree from time to time and place to place. But it was hardly common place that vassals were able to choose their lords freely as you suggest, before or after Gregory VII.
The very fact that the controversy took place is sufficient to prove my point, that neither the church ran the state, nor the state ran the church throughout the Middle Ages.
Actually it does provide a good example that proves that the church from time to time ran much more of the state in particular territories than just religious matters. Other examples during that same exact century can be brought forth showing the secular government controlling the church. The back and forth nature of the chaotic church/government relations of that time only demonstrates that their were super states, that the church from time to time was the super state, that the church at other times was nothing more than a vassal to a non church super state, and still at other times shared power with super states. Your whole idea of a "proto" system of stability did not exist during the times you speak of.
Do not forget though that we are talking about a period of roughly 1000 years.
I'm not forgetting. In a prior reply of mine I asked for some specificity so as to look at this period in the best possible light to your case. Even here, it is impossible to discuss this matter without specifying the years when this 1000 years began and ended. If you would select the best century out of those unknown 10 centuries, then maybe we could better discuss this proto system that I say never existed.