Many a time I realize that I have forgotten to say Maariv in midst of reciting the bedtime Shema.
Halachic Question: Is one permitted to Daven Maariv after saying ברוך המפיל?
Of course you need to pray maariv. Even if we say that the Kriat Shema of bedtime made you quite for the mitsvat KS and Yetsiat Mitsrayim (If you said also Vayomer). If you read in S.A. O.C. 239 and mefarshim, you will see that to say some psalms for shmira after blessing hamapil because of the importance of Shmira. A fortiori, a mitsva as Arvit. 2 brachot before Shema, 2 after it. Even to go at Shtiblech to pray be minyan is allowed. In the Tur & Bet Yosef at the same siman there is no mention of "prohibition to speak", but Bet Yosef mentioned in name of Kol Bo, Rabenu Yerucham and Rokeach, not to eat, drink or speak after KS al Hamita. No mention of Hamapil. No mention of praying or make a mitsva or to go to bathroom.
You need to make a separation between 2 problems, 1.- is the bracha of Hamapil canceled if after it you make something, is it Bracha Levatala and you need to say Baruch Shem Kevod Malchuto... , need you to blessing it next time at the new bedtime. 2.- Is there a cancellation of all obligation through this Bracha.
The second point is obviously not right, because for such a Chiddush we need a proof and nobody in SA treat this. The Magen Avraham told about a man who want to be in marital relationship and advise to say Hamapil after but he doesn't say that marital relationship is prohibited after Hamapil.
The only considerations concerning things after Hamapil are Lechatechilla (is it right to make this a priori). But nothing is written Bedieved (a posteriori).
Arvit is a duty (despite that originally arvit is reshut) and we need to consider it as a duty already at the time you said Hamapil. You cannot cancel a duty.