Learn Russian: Случился форс-мажор, так что мы перенесли встречу. - Force majeure happened, so we postponed the meeting.

Yes, they are nearly interchangeable in this context. However, так что (so / with the result that) is a conjunction that directly ties a consequence to the previous clause within the same sentence. Поэтому (therefore / for this reason) acts more like an independent adverb and is very frequently used to start a brand new sentence.

Why is the verb first in Случился форс-мажор instead of saying Форс-мажор случился?

Putting the verb before the subject is the standard way in Russian to introduce a new, unexpected event. Saying Форс-мажор случился with standard subject-verb order would sound like you were already discussing this specific emergency and are now confirming that it took place.

Does the verb перенесли always mean "postponed"?

Literally, it means "carried across" (the prefix пере- implies moving across or changing state, and нести means "to carry"). While it can mean physically moving an object to a new place, it is the standard verb used for "moving" an event or appointment to a different time or date.

Since форс-мажор is a borrowed hyphenated word, is it indeclinable?

It actually declines like a standard masculine noun, but only the second half of the word changes. For example, if you needed to use the genitive case, you would say из-за форс-мажора (because of force majeure).