1) 1) The opening words of Kaddish, which are “Yisgadal veyiskadash” are based on the words in the prophet Yechezkel “And I became great, sanctified and known to the eyes of many nations, and they will know that I am Hashem!”. (Shulchan Aruch 56)
2) 2) The saying of “Amen yehei shemeih etc.” is more important than answering Kedushah. It follows that if he is in a place where there are many groups of people praying and one quorum is up to “Amen yehei shemeih etc.” and the other is up to Kedushah, that he should answer “Amen yehei shemeih etc.”. In the middle of saying “Amen yehei shemeih etc.”, he should not answer Kedushah. (ibid)
