Learn Turkish: Bu zorunluluk yerine başka bir tedarik planı yapmak durumundayız. - Instead of this obligation, we find ourselves having to make another supply plan.

While yapmak zorundayız means "we are obliged to make" (pointing to a strong, often external obligation), yapmak durumundayız translates more closely to "we are in a situation where we have to make." It softens the obligation slightly by emphasizing that the circumstances are forcing this action, rather than a strict rule or person.

Could we use kalmak here, as in yapmak durumunda kalıyoruz, instead of the "to be" ending (-yız)?

Yes, absolutely. Using the personal ending -yız (we are) just describes the current state ("we are in the position of having to make"). Using kalmak (yapmak durumunda kalıyoruz or kaldık) emphasizes being left in that situation or the change into that state ("we find ourselves / we are left having to make"). Both are standard for this grammar structure.

We have bu zorunluluk followed by yerine (instead of). Does yerine always just follow the bare noun like this?

For regular nouns, yes. yerine acts like a postposition and follows the bare noun (e.g., kahve yerine, instead of coffee). However, if you use a pronoun, it requires the genitive case and personal agreement on the postposition, such as benim yerime (instead of me) or onun yerine (instead of it/him/her).