can i replace the hole (in red) with any random key-hole from another S4, or possibly S5?

 

Yes but it wont work with your key.

 

its the lock cylinder that you want to replace, but it should be feasible to replace it.

 

how would i replace the lock cylinder?

can i simply buy one from a shop to match my key?

 

A locksmith can make one to match your key. Cost more $$ to do it that way. If you buy one from a auto parts store it will come with its own key. Don't know if Mazda would have one and if they did, I don't know if it will come with its own key or if they could use your VIN to match the lock to your key.