I am using Ubuntu 14.04.5 LTS & I am not always connected to the internet for downloading software-packages.  

Suppose, for instance, I downloaded packages like VLC, Banshee etc, using synaptic package manager, and after a month or so I have to format the system. 

So here are my queries :- 

1. How to backup these 2 softwares EARLIER so I won't have to go through the pain of downloading them again?
2. Does synaptic uses `dpkg` or `apt-get` by default? If it uses `dpkg`, how to change it to `apt-get`? Is it recommend to change it to `apt-get` or `dpkg` is good?
3. Is there a way to ensure that after fresh installation of the Ubuntu, Synaptic only downloads the Repositories and not the Updates?

EDIT :- I KNOW THAT `.deb` files downloaded by Synaptic are kept at

```
/var/cache/apt/archives/

```

But you can't just randomly put any deb files there and than sew it in synaptic for installation. I've tried it.

Eg. If I have a package and all its dependencies as `.deb` and to install them I keep them at `/var/cache/apt/archives/` and then open synaptic.
But you see that software won't be visible in synaptic for installation. (won't be in search list)

I want to install backed up packages from synaptic not with 
`sudo dpkg -i *.deb`