1. It is the short form, not the adverb form. You can tell this because the word doesn't have any other word to modify. The word is, itself, the predicate of the sentence ("blank *is* blank" scenario)
For it to have been the adverb form, it would've made sense to translate it with a -ly on the end, roughly speaking.

2. It's hard to tell why your dictionary does this.
If an adjective has one short form - it will have all of them. I seem to have a sneaking memory floating in my head of one or two times I've seen an adjective that had only certain short forms. Either way it's either absolutely incredibly rare or it just doesn't happen.
I think your dictionary might do this, perhaps because of some reasoning involving the Spelling Rules; but that theory is hard to defend.

3. Though a universal rule I think is hard to come by, generally, adjectives that are used exclusively as attributive, particularly long and specific ones, will not have (or really need) short forms.
правительственный - governmental
зубно́й - dental
разгово́рный - conversational, colloquial

(tempting as it may be to extract out English -al as a sign of a non-short-form adjective - that doesn't work all the time)

To some extent, if you can't make short forms out of it, it is because you don't need to.