It was only in the last year or so I discovered outwith was a predominantly Scottish word, I use it all the time, I've probably even used it on this site.
I agree with the previous posters who have said it isn't interchangeable with outside as it does mean something different. You would say "that's outwith my control", "that's outwith the realms of possibility" same as you could change those phrases to "within my control/within the realms of possibility", you would not say "that is inside my control...etc"
Wee=little/small but can also be used to replace cheeky in the way people say "a cheeky Nando's" "a cheeky pint".
"Back of five" is 5:01-5:30, then it turns in to "afore six" 5:31-5:51.
For years us Scottish people have been told that these words are slang/scruffy but they are not, most of the words described that way are, as a previous poster said, Scottish English - aye, naw, bairn/wean, hoose, feart, shoogly are all examples of Scottish English.
There are of course regional differences, even by just a few miles, for example when saying "The children are in the garden",I would say "The children are out the backdoor", that is probably very regional to my area.