Possible to decrypt every fragment of an ecrypted file on its own?
Unless I'm misunderstanding something this should be fairly easy to do.
The scheme of a stream cipher is the following : 

                    Key
                     |
                     v
               +------------+
               |            |
     IV -----&gt; |  Chacha20  +-----+
               |            |     |
               +------------+     |
                                  |
                                  v
MESSAGE ------------------------&gt; +------&gt;  CIPHER TEXT


Thus you have : 
$$K_{stream} \oplus Message = CipherText$$

If afterward you split your $CipherText$ into blocks of $n$-bits. Then you will notice that the first $n$ bits of the $K_{stream}$ are used to encrypt the first $n$ bits of the $Message$ resulting into the first $n$ bits of the $CipherText$ and so on... 

Therefore, in order to decrypt a $block$ on a separated device, you need to know its position into the full $CipherText$ (how many bits are present before), generate the $K_{stream}$ from the $Key$ and $IV$, drop the first $n$ bits of this $K_{stream}$ where $n$ is the position of the $block$ in the full $CipherText$, and then you can start decrypting your $block$.

Remarks:


you don't need to know the length of the full $CipherText$ but only the number $n$ of preceding bits.
you will need this number anyway when you will want to recombine the blocks on a single device.