I planned to install Slackware on my laptop as a dual boot with Windows, then I realized I won't have access to the internet during the installation.

The dorms where I live only allow one single MAC address to authenticate on the intranet, that spot is reserved for my router, but my router is a cheap travel device without ethernet output.

Also, until next week I won't be able to use that access point at all and the only way I have to connect the laptop to the internet is through USB tethering (which depends on some very unstable drivers.. ).

Should I wait to have at least a WIFI connection available or a full off-line installation should work more or less without any big problem? Can I eventually download the additional resources (drivers.. ) elsewhere and transfer them to the laptop by USB key?

The dvd isos are designed to install Slackware fully without requiring a network connection. The default kernel has pretty much all drivers built in, however, if your devices are newer and are not supported by the kernel (some video cards won't have decent performance without newer drivers and some wifi cards are unsupported without firmware), you can download the drivers separately, and transfer them to your computer using any number of methods, including USB drives. But you should be able to get to a desktop in most cases without ever needing to connect to the network.

I would also recommend going to your favorite mirror and grabbing all the patches for your version. There have been a lot of vulnerabilities in various software that Slackware includes, and Pat has patches to make sure you're running the most up-to-date version of Slack possible. You can transfer these by USB also.

However, if you're not connected to the internet, most of the vulnerabilities that have been discovered won't be an issue, but once you are connected, you could be a target without them.