How should one approach HaMapil when there is a chance of tashmish being requested? Can one engage in tashmish after HaMapil, especially since talking will be necessary?
This seems to be a bit of a judgement call, and it depends on how you feel. If you suspect that it might happen, then don’t say hamapil yet, but if it is just because there is a chance… then you should say hamapil. If it is requested, or you otherwise feel that it is needed, then you do what you are supposed to, even though you will be talking out. The poskim say that we are allowed to talk out after saying hamapil for important things, if he needs to say Asher Yotzar, Sefiras Haomer, or daven maariv.
Mekor Chaim HIsorirus Teshuva 1-114 Pri Hasadeh 1-93, Beer Moshe 1-63, Halichos Shlomo (Tefillah) 13-16 ftnt. 24, PIskei Teshuvos 239-3.