Dear Tzungkuen,

t> etam. yajetha medhaavii, eso ya~n~no mahapphalo
t> may be translated as follows

t> The wise may perform this sacrifice, and this sacrifice is bearing much fruit.

t> yajetha: (yajati)opt.3.sg.P. meaning 'to sacrifice'.

I think that optative here is a polite imperative:

It would be good if the wise one will perform this sacrifice, it bears
much fruit.

t> But I don't know how to translate the next stanza
t> etam. hi yajamaanassa, seyyo hoti na paapiyo

eta.m.: this (acc/gen m. or f. of eso/etad/esaa)
t> yajamaanassa: (yajati) ppr.m.sg.gen/dat. 'performing a sacrifice'
t> seyyo: (seyya) ? meaning 'better'.
t> hoti: meaning ЎҐisЎ¦
t> na: meaning ЎҐnotЎ¦
t> paapiyo: (paapiya) ? meaning ЎҐworseЎ¦ 453

Indeed (sacrifice) of such sacrifice-performer is better, not worse.

With metta,
Dimitry