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How would it be proper to characterise (adverbially or adjectivally) longus usus, opinio juris so as to mean a belief of law (belief of a legal requirement) in long use holding uninterrupted and uniformly?
Would opinio juris, longus usus tenaciter servanda, be proper? What would be the most idiomatic translation of "a belief of law in long use holding uninterrupted and uniformly?
No, that translation is not grammatically valid. It means roughly "belief of law, long use, to be saved firmly" but it is somewhat incoherent. Let me go through a translation process step by step.
As you seem to know, opinio iuris is a fixed expression and we can of course start with that. The modifiers will probably not be parts of standard legal Latin terminology, so we have to coin something that works. (This of course changes if anyone knows existing terms that could be used or modified neatly.) The noun opinio is feminine.
A couple of words come to mind for continous or uninterrupted: continuus, assiduus, perpetuus. The first two come from the verbs continere and assidere. I will go with perpetuus. Check an online Latin dictionary for details on the nuances or find a related expression from legal Latin and choose accordingly.
For "uniform", I would go simply with uniformis, which quite literally means "one-formed". If the same belief is held in the same form in many places or times, this sounds like the perfect form.
I would go with simply opinio iuris perpetua et uniformis. I think perpetuus conveys "in long and uninterrupted use" very well, so I would not add separate words for "long" and "use".
Sebastian Koppehel's comment suggests that longus usus ("long use") is a common legal expression. Therefore it would make sense to replace my perpetua with in longo usu. If that sounds preferable to you, then I suggest opinio iuris uniformis et in longo usu.
Use of the gerundive servanda implies that the opinion must be saved. If it is a description rather than an instruction, the past participle servata is more apt. But I don't find it necessary at all, as the two adjectives contain the message alone.
