word usage - Either in a negative sentence

Do the following sentences present acceptable usages for either?

1. Either this or that fruit is not tasty.

I do understand I should have said that differently i.e. with a positive statement:

Neither this nor that fruit is tasty.

But I am particularly interested in the negation. Some more examples for consideration:

2. 
   

   Either fruits are not tasty.

   
   


3. 
   

   Either students will not be restricted.

   
   


4. 
   
   

   Either of the lakes are not polluted.

   
   


5. 
   

   Either paths have not been taken.

   
   

P.S. I am not interested in these classic usages for either in a negation:

My pal is sober and I am not drunk either. [hic]

We cannot use either copybook.

Answer

either in the pattern you have provided in your example sentences refers individually to two valid alternatives. A valid alternative is one having the required qualities.

We can take either road.

Either road will take us where we want to go.

Either will take us there.

For that reason, it is an inherent contradiction and ungrammatical to use either as the subject of a predicate where a valid alternative is said not to possess a required quality.

Either road won't take us where we want to go. ^{ungrammatical}

The correct form is

Neither road will take us where we want to go.

Neither refers individually to two invalid alternatives.