When can you start answering back Aliechem Shalom meaning can you answer if you are in middle of the first Bracha?
This should be the same as in the middle of the brachos of shema. Strictly, the halacha would allow him to respond to someone who he reveres or fears, but only if this is the first time bumping into him and he is the type that would care, so as to avoid hatred. (Shulchan Aruch O.C. 67:1; M.B.2)