i think 14" tyre cost not more than 200/pcs...

Dont use the spare tyre just use the patched tyre and change it to front if it is on rear. If a nail can easily poke your tyre, it is one of the indicator that your tyre is worn out and you may need to check if it need to be replaced.

left right different tread normal drive nvm lo for me.. but if high speed & cornering beware a bit lo...
else suggest u go buy a set of 2nd tyre which those guy upgrade their rim... these tyre most they left 70%~80% tyre & price is half of new set... it shall save back ur pocket a lot if u don mind~~

As I stated before, if you got some extra money go change both tyres. Pls dont buy 2nd hand tyre for 1 side only as you need to change the other side too.

1 nail may be a bad luck but 2 nails? A call for new tyres. You can go tampal this tyre but you will see another nail either on the same tyre or on other side of the tyre.

Dont forget to put the new tyres at the back and old ones to the front.

Some mechanics wont advice you to do this but this is the logic, your back tyres are 4 years old and if you dont rotate your tyres to the front your back tyres will have 4 more years or more. Im sure you dont want to use 8 years old tyres... dont you?