See SA O"Ch 54:1,
Birkas Yishtabach is deemed S'muchah to Baruch She'amar.
MB 1 says this is despite the intervening p'sukkim [of P'sukkei d'Zimrah] - just as it is so for Sh'ma
So far so good.
Then M"B adds same for Hallel - citing the P'ree M'gaddim in Shaar Hatziyyun 2.
How can the last b'rachah in Hallel be S'muchah to a B'rachah "K'tzarah" at the beginning of Hallel? This seems quite different than the case of Baruch She'amar and Yishtabach where the first b'rachah is a B'rachah Aruccah. It appears that a b'rachah has'muchah follows a b'rachah arukkah and not a K'tzarah.
Even the case of Yishtabach does not seem so absolute.
At the end of the seder some are m'sayeim with Yishtabach while the others are m'sayeim with Y'hall'lucha, with neither one being s'mucha to an opening b'rachah.
My Chaveir Responds
«Tosafos on Kesuvos 8a explains that a beracha ketzara can also generate. a semucha lechraverta , but saying the two back to back would create the. wrong impression that it is one long beracha . It follows that here where Hallel is said between them there is no such problem.»
«This is addressed at length by Tosafos to Berachos 14a. See there for their conclusion that this beracha on Pesach night is a "hodaah be'alma" and so it does not need to open with a baruch.»
Note the answer to #2 DOES suggest that Birkas Hasheer - at least at the end of the Seder - is a hoda'ah b'alma which matches my original point of view.