All of these sentences are grammatically correct, but you won't hear the last one very often, and probably never* encounter the first one. They mean different things.
☼¹ I feel sleepy very much.
I feel sleepy too much.
These are possible ways of referring to how often someone feels sleepy. That is, how frequently. There is some of the statement left off, which would make these sentences:
☼ I feel sleepy very much [of the time].
I feel sleepy too much [of the time].
When someone says “too” they are suggesting an excess beyond an acceptable level. In this case, the acceptable level would be feeling sleepy at regular times or just at the end of the day. The following dialog is possible:
“Why don’t you like taking that medication?”
“I feel sleepy too much (of the time).”
“Too” is also used in one of your other examples:
I feel too sleepy.
Again, someone saying this is implying a tolerance limit that has been exceeded. Someone might make this declaration as they left the company of a group late at night, and the implication would be that they are too sleepy to continue doing what they are doing. More likely you’d hear it in situations like this:
“Why don’t you go with us to the nightclub?”
“I’m too sleepy.”
Most people would say that they are sleepy (not that they “feel” sleepy) unless they are describing something that is similar to sleepiness, especially something externally imposed. c.f.:
“Is the nitrous oxide flowing?”
“Yes. I feel very sleepy.”
More likely people who are very sleepy in normal situations will just say “I am very sleepy.”
1 ☼ = descriptively ungrammatical (violates practice, but not rules).
Both examples are grammatical. In ① and ②, both either and too are working as adverbs.
Adverbs (and more generally adverbial phrases) can complement verbs, adjectives or, as in ① and ②, clauses.
I am unable to hear either.
"Either" in the above sentence complements the whole clause "I am unable to hear". It is used to indicate that the clause is adding some information to the preceeding sentence. As noted by the OP, "either" is used for this purpose only when both sentences express negation. In ①, "unable" provides this negative meaning in both sentences.
Other examples of this usage of either (taken from the dictionary linked above):
You don’t like him, do you? I don’t, either.
It won’t do any harm, but won’t really help, either.
I was too tired to go. And I couldn’t have paid my way, either.
If any of the sentences, as in example ②, does not express negation, then the adverb "either" is infelicitous, and "too" should be used instead:
Note that both "either" and "too" link one clause with the preceding sentence, and hence, there must be a reason to link them.
In the case of ①, both sentences follow the same pattern: "I am unable to".
In the case of ②, more context is needed to make this reason patent, e.g.:
Yesterday was a terrible day: I felt humiliated at the party last night. I was unable to sleep well, too.
Best Answer
No, a double negative would be something like this:
Which is really saying "we are not going to no party" which, according to the sentence, means they are going to a party. See this Youtube video for additional examples of double negatives.
The plan is complicated enough so that we must involve the "interests" and "local institutions" that were previously mentioned.
The long-winded sentence is basically saying that if we do not involve the people and institutions that will be affected by the plan, it will be a bad thing since the plan will affect so many others.