Translation:The boy eats the good fruit and is happy.
I'm still not sure about the difference between Gerpa, Gerpi, Gerpe and Gerp(ii)
Gerpa and Gerpe are singular, Gerpi and Gerpī are plural - nominative and accusative, respectively :)
Could "Taoba syyz gerpe ipradas" also mean "The GOOD BOY eats the fruit"?
Yes, but given the ambiguity the default is to consider the adjective to be modifying the noun after it rather than before.
Why could this not also be "Taoba syyz GERPII ipradas se kirine issa"? Couldn't both gerpe and gerpii fit, since they are both accusative?