"Mandatory" means that the thing must be done due to some reason or rule.
These courses are mandatory.
That is, you must take the courses or else you fail.
Wearing helmets was made mandatory a few years ago.
It is a rule that you must wear helmets.
"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.
The volunteers' help was indispensible.
The mission would have failed if the volunteers didn't help.
He made himself indispensible to the parish priest.
He was deemed a must-have person because of some good quality.
These great tools are so versatile that they are indispensable.
The tools are so good that you shouldn't be without them.
To sum up, these two words are not interchangeable - they have their own distinct meanings.
North America has what we call the father-bother merger, where /ɑ/ and /ɒ/ often end up as the same sound, oftentimes /ɑ/.
So thus /ɑ/ would be a better choice in North America.
Merriam-Webster's system is a bit unique. I don't like to use it in a linguistic discussion. /ä/ = IPA /ɑ/.
IPA is the more universal option for phonetics as a science, even though several symbols will trick English speakers (e.g. /a/, /e/, /o/, /y/ don't exist alone in major English dialects, and /j/ is not the English J, it is the Y consonant, and people love to eschew the standard rhotic sign in English for the trill symbol /r/, which does not exist in most English speech).
Across the pond, Received Pronunciation British and other dialects don't merge "father" and "bother" vowels like that, thus you get /ɒ/ to accommodate (by the way, /ɒ/ is the rounded version of /ɑ/.) Which transcription is "correct" would depend on the dialect of focus, in this case American English.
Best Answer
Nearly all speakers give both words the same pronunciation—but not all speakers use the same pronunciation. Some say fē-än-sā′ for both, some say fē-än′-sā for both. The difference is just which syllable is stressed.
Here's something about the pronunciations of these words that you won't find in dictionaries. Educated people often see these as French words being used in English, so they put the stress on the last syllable and spell them with the accent: fiancé, fiancée. Putting the stress on the third syllable runs counter to the normal sound patterns of English, giving these words a foreign sound—specifically, a French sound. Uneducated people often don't know that these are French words, or don't care, so they make them conform to the normal patterns of English: they put the stress on the second syllable and don't write the accent. Some middle-class people, trying to appear more cultured or upper-class, exaggerate the stress on the final syllable. Some stress the second syllable and lengthen it in order to sound sophisticated. Each pronunciation can sound silly or pretentious to people who use the other pronunciation.
So, people's pronunciation of these words is a not-entirely-reliable indicator of social class, at least in the United States. Paul Fussell talks a little more about this kind of thing in the book Class. I don't agree with all the details of what he says there; perhaps some of these class-markers have shifted since he wrote the book (1992). But the principles still hold. Fussell hints that boyfriend and girlfriend are plain-spoken synonyms. The meaning is different, because these don't mean that the person is engaged to be married, but it's fine to refer to a person's fiancé or fiancée as their boyfriend or girlfriend.
Gradual absorption of foreign words
The moral of this for someone learning English is that sometimes English adopts a foreign word in gradual stages. At first, the word is perceived as still foreign even though it's in use within English sentences. For example, chargé d'affaires is usually italicized in writing and pronounced in French style. If the word becomes more fully absorbed into English, people drop the italics and adjust the pronunciation to fit English patterns. If the word has diacritical marks, eventually those will be dropped, too. However, fiancé is under a lot of pressure to retain the accent because the final e is pronounced; without the accent, the spelling would suggest that the e is silent. (Not so with fiancee.)
During this process, different people treat the word differently: some treat it as fully foreign, some as partly foreign, some as fully absorbed, some misunderstand it, etc. And during this process, these differences become an opportunity for people to demonstrate—or try to demonstrate—their familiarity with a respected foreign culture. Especially if the word comes from French, people can try to use what they think is a more authentic pronunciation to gain social status. Different people perceive all this with different levels of acuity, resulting in the differences in pronunciation and perception described above.