Copying applications and data manually/Wifi issues with El Capitan
Can you copy your data and applications?  Yes.

Should you?  I most definitely don't recommend it.  It's a extremely time consuming processes and there is no guarantee that what you copy over will work properly.

While I personally believe it is beyond unacceptable that users have been suffering through WiFi issues with each iteration of OS X, it's not feasible to keep upgrading/downgrading operating systems in the hopes of being able to connect.

What I would do (and have done for others) is get a USB WiFi adapter.  If you don't need 802.11ac but 802.11n is more than adequate, then have a look at this adapter

What you do, is disable the onboard WiFi and move this one up in the priority list under Network Preferences.  

This will solve your WiFi issues without having to go through the trouble of backing up and downgrading your OS