Rather than trying to devise a rule to automatically follow, one should aim to understand what comparatives and superlatives mean. The comparative is used to identify an ordering of two things on a scale, and has to be interpreted relative to a context that has exactly two things. The superlative applies to any context, whether there is 1, 2 or 200 things to choose from: it does not mean that there have to be be at least 3 candidates, and "most" would be satisfied if there is only a single candidate. What is most important is being clear about that set of candidates, and this is accomplished by carefully specifying the set of possibilities. (Separate attention has to be paid to guaranteeing that there are any candidates, and if you want to specify that at least one truck must be provided, then you should say that, and not leave it to inference that "the heaviest truck" suggests that there should be at least one truck).
Assume a contract where A transports goods from Paris to Rome, with an unknown number of routes that lead to Rome. The intent is to require taking the shortest route, since the customer pays by the mile.
A shall transport goods by the shorter route from Paris to Rome
A shall transport goods by the shortest route from Paris to Rome
These clauses fail to specify a pool of routes to consider. Clause 2 implicitly says "out of all of the possible routes, pick the shortest", the main problem being that this is air transport (saving hundreds of miles compared to surface roads). This can be remedied by specifying a type of route (e.g. "via E-grade highways"). Once you've narrowed it down that way, it does not matter if there is only 1 such route, 2 of them, or 12 routes: A must pick the shortest. Clause 1 is open to the ludicrous interpretation that the route cannot be the longest possible route. The route via Hammerfest, Irkutsk, Phnom Penh, Madras, Capetown and Athens is shorter, compared to Hammerfest, Phnom Penh, Irkutsk, Athens, Capetown and Madras (in that order). Saying that the route must be "shorter than some other thing" is not what you want: you want "shorter than all other things".
A comparative (with an indefinite article) can be sensible when the set of possibilities is already constrained, if you are stating an "exception" to a more general rule. If the set of routes is already limited to those which employ E-system highways for at least 75% of the distance, then you can "exceptionally" allow selecting a longer route: "A may take a longer route if that route reduces toll costs by at least 10%". This, incidentally, allows A to select a route via Irkutsk, if that reduces tolls, so you still need a superlative to select which of those longer routes may be taken.
In other words, there is little reason to use a comparative, no reason to follow such a rule on word use, and every reason to pay attention to how the interpretation of clauses is affected by context.
