To answer the first question, it is not in dictionary form because in formal Japanese, or generally often in writing, it is customary to connect two sentences using the "pre-masu form" that is, the -masu form without the ”ます” (for example: 食べる → 食べ、 行く → 行き、 and in your case 転がる -> 転がり).
So in your case you could see this as the equivalent of putting an "and" after the verb in English.
To answer the second question, as pointed out in a comment one way of thinking about the -てしまう form sometimes is to see it as "Ended up...". Moreover in this case we could imagine that "falling down a hole" represents a somehow undesirable situation since after that point the rice ball might be lost. This however, might depend on the context of course.