1. The mountain is so high. We had better go for it.
1-1. The mountain is so high. We had better not go for it.
2. The mountain is so high. We had better go from it.
3. The mountain is so high. We had better go away from it.
4. The mountain is so high. We had better go around it.
5. The mountain is so high. We had better go at it.
6. The mountain is so high. We had better not go at it.
7. The mountain is so high. We had better not go to it.
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Which ones are grammatical?
None.
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions
Notice in the chart that "so that" is considered a subordinating conjunction. This is true whether they are together in the sentence or separated. Therefore, all your sentences need to be combined and follow this pattern:
The mountain is so high that we had better go for it.
Out of the provided sentences, the grammatical ones are:
1. The mountain is so high. We had better go for it.
1-1. The mountain is so high. We had better not go for it.
4. The mountain is so high. We had better go around it.
7. The mountain is so high. We had better not go to it.
To determine the grammaticality of a sentence, it is important to consider the verb phrase and any prepositions used. In this case, the phrases "go for it," "go around it," and "go to it" are all grammatically correct expressions. However, phrases like "go from it," "go away from it," and "go at it" do not convey a correct meaning in the context of the given sentence. Therefore, sentences 2, 3, 5, and 6 are not grammatical.