Shulchan Aruch, OC 236:3:
If someone discovered a group that had already read [the evening] "Sh'ma" and is about to say amida [of maariv], then he should say amida with them and then say "Sh'ma" with its blessings.
- Is there any reason for him to say or hear "Bar'chu" before he says the blessings of "Sh'ma"? Any reason not to? Sourced answers only, please. (I know OC 69 discusses the somewhat similar rule of pores al "Sh'ma", for which one says "Bar'chu", but I'm looking for a source that explicitly says whether or not to say "Bar'chu" in the case discussed in OC 236:3.)
- The community will be saying kadish and "Alenu", which under normal circumstances a bystander would respond to. But our latecomer is between sh'mone esre and the blessings of "Sh'ma". Must he reply? Must he avoid replying? In general, is there any reason for him to avoid interruptions between sh'mone esre and the subsequent blessings of "Sh'ma"? What, for example, about chatting with his friend? (I know there's a general rule of avoiding delay before saying "Sh'ma" (Mishna B'rura 235:17), but I'm asking about cases where the general rule permits delay but there may be some rule specific to our latecomer's situation that bars interruption/delay.)