It's not easy to tell because it doesn't make a difference.
Let's look at the first sentence on your list:
The pension plans involve very little risk for employers.
First, we'll drop the prepositional phrase entirely. Next, we'll separate the keywords in question, "involve" and "risk", in this case by casting the clause in the passive voice. Then, we'll reintroduce the prepositional phrase such that whatever it modifies is clear. Finally, we'll compare those results with the original.
The pension plans involve very little risk.
Very little risk is involved.
Very little risk for employers is involved / Very little risk is involved for employers
Regardless of whether the phrase in question is part of the subject or part of the predicate, the overall meaning of the clause remains the same. This exercise produces similar results for the remaining sentences:
- a reserve for losses is created / a reserve is created for losses
- returns for investments are required / returns are required for investments.
- enough capital for the risk is kept / enough capital is kept for the risk
- the policies as collateral are used / the policies are used as collateral
- a maximum level for the size is specified / a maximum level is specified for the size
- contracts for 10 stations have been introduced / contracts have been introduced for 10 stations
- limits for traders are defined / limits are defined for traders
- the distribution for such products differs / the distribution differs for such products
You're right. There is a technical, grammatical ambiguity between, for example, risk for employers and involvement for employers. In fact, this technical ambiguity reaches an even greater depth in sentence 5: collateral for loans / policies for loans / used for loans.
However, there is no semantic ambiguity.
In every one of these examples, by the time we're parsing the meaning of the entire predicate, it makes no difference whether the prepositional phrase in question modifies the complete predicate, only the verb, only the direct object, or only one component of that direct object. In our passive voice transformations, it doesn't even matter whether the phrase modifies the subject or the predicate.
In a sense, it's not even possible to modify only the predicate, or only the verb, or only the direct object, or only one component of that object. Semantically, the meaning of any such modification is inevitably carried up to the level of the complete clause.
Since it doesn't make a difference, the simplest choice is likely the best choice. I would say that the smallest and closest candidate is the simplest choice. For simplicity's sake alone, I'll parse sentence 5 such that "for loans" modifies "collateral". When I do this, I don't insist that the phrase "for loans" must be adjectival in this clause. I only claim that it is a convenient (and harmless) label.
It may be important to contrast these sentences with sentences where the grammatical ambiguity does make a difference.
- One morning, I shot an elephant in my pyjamas. How he got in my pyjamas, I don't know. --- Groucho Marx
In the context of the joke, "in my pyjamas" modifies the noun "elephant". In the context of ordinary life, "in my pyjamas" should modify the verb "shot". The joke works because our ordinary-life expectations of an adverbial prepositional phrase are overturned by the subsequent adjectival interpretation.
Ignoring jokes, this kind of real ambiguity does exist.
- I threatened the man with a gun.
Without more context, two clear possibilities exist:
- I used a gun to threaten the man.
- The man with a gun is the person that I threatened.
Unlike all of the example sentences in this question, the two possible interpretations yield two completely different meanings at the level of the clause. Here, because it does make a difference, knowing the meaning of the clause does let you determine whether the phrase is adjectival or adverbial.
We can even use the passive-voice technique to highlight the difference:
- the man with a gun was threatened / the man was threatened with a gun