user76495: We assume that trying to mine a block as of 2018 is like digging for gold with a plastic spoon. There are chances but remote if any, of success.
That being said, if someone can get the info from Getblocktemplate and decides to mine a block for luck and happen to mine a successful block that meets the target requirements.
Is it enough to submitblock with the appropriate info like this:
bitcoin-cli submitblock 000000205badedd2461eabc2c47aeac53a784fa7fc3b79f2566f27000000000000000000b3433d3f19fb599c34704bd2dede4aeab7d9f1e2658de3b30e6004d44b985411d1637c5af8e961173a3a8db3 1 
being the first argument the message and the second a dummy that is being ignored by the network.
1. Should there be any response?
2. Should there be any quotes?
3. Why it always return 
error code: -22
error message:
Block decode failed

4. Is there a way to send the block to a relay node that will spread it fast?
Any help would be appreciated and please do not mention again that it is impossible to mine a block alone, it is already known.

user75001: If you run:
>bitcoin-cli help submitblock

It provides some information and says in part:

See https://en.bitcoin.it/wiki/BIP_0022 for full specification.

Which states for submitblock:

This method MUST return either null (when a share is accepted), a String describing briefly the reason the share was rejected, or an Object of these with a key for each merged-mining chain the share was submitted to. 

Please see this question (findable via a search for 'submitblock') for further information.
It looks like there should be quotes.
When a block is successfully submitted it should be broadcast automatically but, it should build on the tip otherwise it is irrelevant.

76495: <p>We assume that trying to mine a block as of 2018 is like digging for gold with a plastic spoon. There are chances but remote if any, of success.</p>

<p>That being said, if someone can get the info from Getblocktemplate and decides to mine a block for luck and happen to mine a successful block that meets the target requirements.</p>

<p>Is it enough to <code>submitblock</code> with the appropriate info like this:</p>

<p><code>bitcoin-cli submitblock 000000205badedd2461eabc2c47aeac53a784fa7fc3b79f2566f27000000000000000000b3433d3f19fb599c34704bd2dede4aeab7d9f1e2658de3b30e6004d44b985411d1637c5af8e961173a3a8db3 1</code> 
being the first argument the message and the second a dummy that is being ignored by the network.</p>

<p><strong>1. Should there be any response?</strong></p>

<p><strong>2. Should there be any quotes?</strong></p>

<p><strong>3. Why it always return</strong> </p>

<pre><code>error code: -22
error message:
Block decode failed
</code></pre>

<p><strong>4. Is there a way to send the block to a relay node</strong> that will spread it fast?</p>

<p><em>Any help would be appreciated and please do not mention again that it is impossible to mine a block alone, it is already known.</em></p>
 75001: <p>If you run:</p>

<pre><code>&gt;bitcoin-cli help submitblock
</code></pre>

<p>It provides some information and says in part:</p>

<blockquote>
  <p>See <a href="https://en.bitcoin.it/wiki/BIP_0022" rel="nofollow noreferrer">https://en.bitcoin.it/wiki/BIP_0022</a> for full specification.</p>
</blockquote>

<p>Which states for submitblock:</p>

<blockquote>
  <p>This method MUST return either null (when a share is accepted), a String describing briefly the reason the share was rejected, or an Object of these with a key for each merged-mining chain the share was submitted to. </p>
</blockquote>

<p>Please see this <a href="https://bitcoin.stackexchange.com/questions/43105/how-to-use-bitcoin-cores-submitblock-method">question</a> (findable via a search for 'submitblock') for further information.</p>

<p>It looks like there should be quotes.</p>

<p>When a block is successfully submitted it should be broadcast automatically but, it should build on the tip otherwise it is irrelevant.</p>
