MB2 151a: Davening Shemoneh Esrei Before Shema by Ma'ariv (236:3-4); Why No Tachanun or Chazarat Hashatz at Ma'ariv (237:1)

If you come late to Maariv should you daven shemoneh esrei with them and say shema afterwards? Why is there no chazarat hashatz or tachanun at maariv?