Question 1)

Челове́ческому гла́зу э́то недосту́пно.

In this sentence is недосту́пно an adverb or a neuter short form of the adjective недосту́пный ?

Question 2)

My dictionary often gives only say the masculine and feminine forms e.g.

хороший (-, а)

but sometimes if gives masculine, feminine, neuter and plural forms e.g.

храбрый (-, -а, -о, -ы)

does this mean that is there is no e.g. plural form in those cases where it is not given?

Question 3)

Some adjectives have a short form and others don't. Is there any way to know an adjective can be used in the short form other than just knowing?

спасибо