May/should one answer to Kaddish in the middle of Hallel (and in Yehallucha)? Thank you

Shalom, Thank you for your question. The laws of interrupting Hallel are generally that one should be careful and not make any breaks from the start of Hallel, until the end of the closing bracha. However, if one hears a blessing, brachu, kedusha, or kaddish, one may answer – except for saying “Baruch u’varuch shemo”, which should not be said. Also, if one is saying the closing Yehallucha bracha and you hear the chazan finish his bracha, you do not answer amen. (source: Eishay Yisrael, (Hebrew) chapter 39. 46) Blessings.