If one has not said ha-Motsi before eating bread( of the amount for which Birkat ha-Mazon is required), is one still required to say Birkat ha-Mazon after eating?
Yes, the brachas don't always go hand-in-hand.
For instance, let's assume (but check with your rabbi) that one piece of pizza is a "snack", and two makes a meal. Kitzur Shulchan Aruch rules (about pseudo-bread items, let's assume pizza is such an item) that if you decide you only want one piece of pizza, you make a mezonot then eat it. If you then change your mind and want only one more, you don't make a hamotzee for just one piece of pizza, so you make a new mezonot. But then you've eaten two pieces, so you say birkat hamazon.
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I would add that Birkat Ha'Mazon is the only blessing that is דאורייתא (directly from the Torah) as opposed to the rest (incl. ha-Motsi) that are דרבנן (added by our sages).
With this difference it is obvious that we would do the דאורייתא even if we didn't do the דרבנן.
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To answer this in a different derech from Shalom's excellent answer:
Birkat Hamazon is an independent obligation from Hamotzi. If you forgot to say a beracha before eating bread, you'd still be required to say Birkat Hamazon when you finished.
Note that there's no minimum shiur required for saying Hamotzi. You need to say Hamotzi on even the smallest crumb of bread. There are minimum shiurim for both washing (one k'beitzah) and Birkat Hamazon (one k'zayit).