I believe easy-to-understand is an example of a compound modifier: https://en.wikipedia.org/wiki/Compound_modifier
I agree that your other two sentences have pretty much the same meaning as the original one. I would rephrase the "are..." part to sound more natural and "correct" as follows:
The easy-to-understand questions are so important for making our decisions!
"important for" and "important to" are both valid terms but have slightly different meanings - "important to X" is like "something X has a personal interest in," while "important for X" is more like "something X needs for a particular purpose." Here I think "for" makes more sense. Both "important to" and "important for" take a direct object that is a noun phrase - so here it should be the gerund "making". Not "make" - "to" is not an infinitive marker here and this should not be a regular verb.
I believe there is a rule that says that "which" denotes a nonrestrictive clause and omitting it doesn't change the meaning of the noun phrase. It normally requires commas separating this nonrestrictive clause from the rest of the sentence. In contrast, "that" denotes a restrictive clause - essential to the meaning of the noun phrase and the rest of the sentence. So here's how you could use these rules in your sentence:
The questions, which are easy to understand, are so important for making our decisions.
The questions that are easy to understand are so important for making our decisions.
I am not sure exactly what you mean by this sentence so I am not sure which of the above would be more appropriate. Are all questions easy to understand? In that case, use "which". Are only some questions easy to understand, and are those the ones that are important for making decisions? In that case, use "that".
And the syntax "the questions, easy to understand,..." also seems grammatical. This would be the same as using "which".