I think your questions are based on an over-valued notion of stressed syllables being necessary to validate schwas.Dear members and teachers:
''We need a stressed syllable for a Schwa sound, even in function words; otherwise there will be no Schwa''.
This rather Zen-like assertion appeared a few weeks ago. I'm sure there's a flaw in it. If we are talking about words, it's wrong. There doesn't have to be a non-schwa sound in a word for it to have a schwa sound. Maybe having the non-schwa sound somewhere else is sufficient?
Function words unlike content words are those that do not belong to dictionary words and are less important than content or lexical words in a text or conversation. They are grammatical words connecting with content words.
"Dictionary words" is a new concept to me. I'm pretty sure that these short functional words that often contain a schwa and no other vowel are given their due place in dictionaries. "Function words" is suitable.
Although Schwa sound does not ocurr in monosyllable words [It does!], function words are monosyllable words which its vowels are reduced to Schwa for fitting the rhythm in the spoken English. Hmm. When is a schwa not a schwa?
As my quote estates (states?, asserts?), a stressed syllable is needed for a Schwa (not necessarily in the same word). In spoken English sounds are connected together as one sound, so in that connection of sounds there are interval of stressed and unstressed sounds.
1°) Is there a stressed syllable prior to or after the (vowel of the) function word that becomes Schwa?
Just a small quibble - the only word that I can think of that can become a schwa is "a".
No, it's not necessarily true that a schwa is either preceded or followed by a stressed vowel in the adjacent syllable. Several consecutive schwas are common.
2°) If there are two function words consecutively (of the; for the; of a; for a), is the next syllable to be the stressed one?
Not necessarily, again. Do you mean two consecutive schwa vowels? If the following word begins with a schwa, it keeps it.
"I reached for the potato". /fə ðə pəteɪtəʊ/