"cad is brí le "mate" as Gaeilge?" means "what does "mate" mean in Irish?". That's the wrong question, because "mate" doesn't mean anything in Irish.
"Cad é an Ghaeilge ar "mate"?" is how you ask "What is the Irish for "mate"?"
"a chara" or "a chomrádaí" are probably the best choices in this particular context.
No, i and sa aren't interchangeable.
Most countries require a definite article, so you have to use sa with those countries, or, as in this case, san when the country starts with a vowel.. A small number of countries don't require the use of the definite article - Ceanada is one of them, so it's just i gCeanada.
The most common countries that don't require the use of the definite article are Éirinn, Sasana, Albainn, Meiriceá and Ceanada.