If the concertina has a tendency to open a bit when not compressed, what's the problem with that? I never close my bellows fully when I am playing. Really you want little resistance either direction;  the point is to have the bellows move in and out freely.  I am sure there are no peer review studies but surely someone must have first hand experience. I've only been playing for about 50 years (EC for ten and Anglo for 40) and haven't had any bellows problems to speak of that were caused by not compressing the bellows. Not having the bellows compressed seems to me would make the concertina better able to withstand a blow to the ends since there is more give. That's why I fancy a well made canvas bag rather than a hard case. If you drop the bag more of the shock will be absorbed by the bag than if you were carrying the concertina in a hard case that had no give to it.