In a "subject is complement" sentence, it is possible for the subject to be singular but the complement to be plural
These potatoes are my dinner.
The "rule" is that the verb should agree with the subject (potatoes) not the complement "dinner".
In "There is/are" sentences, the subject is the pronoun "There" and the complement determines the form of the verb:
There is one potatoes/ There are many potatoes.
Similar exceptions apply in questions, with interrogative pronouns that don't have grammatical number.
When the subject has become separated from the verb and complement it is common and at least tolerated that the subject is forgotten. This is why you might see (2) in natural English. The Subject "risks" is a far from the verb "is/are", but the verb is close to the complement "subject". It is natural (but logically incorrect) for the closer phrase "a subject up for debate" to influence the choice of verb form, and many speakers will naturally choose "is".
This is not a rule that is easy to apply, it is more a common mistake that doesn't cause confusion, and so has become, tentatively, part of English.