dessert, /ə/

[raindoctor]

That makes no difference. The same vowel is heard in, for example, before. For some speakers, that vowel is indeed /ɪ/.

"A --> B" doesn't rule out "C --> B".

[raindoctor]

How do I know to stress or 'unstress' a vowel?

Stress dictates the quality of a vowel. Take a nonsense word like catak.

'catak = cat-ak
ca'tak = ca-tak

See the difference in the syllable structure, stress and vowels

[5jj]

'catak = cat-ak
ca'tak = ca-tak

See the difference in the syllable structure, stress and vowels

Unfortunately, your post says nothing about how the vowels are pronounced.

@ pizza. Some languages are pretty consistent in that a certain syllable (for example the first, the second, etc) is always stressed. This is not true of English. We have no way of knowing how a new word in the language is stressed until we hear it used. Once a word has been used enough for dictionaries to record it, we can then check there..

[5jj]

"A --> B" doesn't rule out "C --> B".

Sorry, but I do not see the relevance of this - indeed, I am not sure that I understand it.

You wrote, "Both /d/ and /s/ are alveolars. Therefore, your hearing /ɪ/, instead of a schwa, is reasonable". The use of 'therefore' suggests that it was the presence of the alveolars that caused /ɪ/ to be heard. I pointed out that this was not so; /ɪ/ can occur between bilabial and labio-dental (and other) consonants.