Merriam-Webster transcribes as /əw/ what the vast majority of other lexicographers transcribe as /u/ or /əʊ/ (or /oʊ/ or /o/) in an unstressed position before another vowel. Jack Windsor Lewis wrote:
In the new 1961 edition of Webster’s Third New International Dictionary the leading American pronunciation lexicographer of his day, Edward Artin, included an innovatory style of transcription for words like obstruent and strenuous etc. He explained in that work (at p. 37a) that he favoured transcriptions like \ˈstrenyəwəs\ because he felt ...“a very strong conviction that ... such pairs as” silhouette/Scylla wet were “exact rhymes” and that this clearly indicated in consequence that his new transcriptions of such items were more satisfactory representations of the normal pronunciations of such words.
In writing silhouette in a phonemic transcription one’s choice is essentially between /sɪl.u.`ɛt/ and /sɪl.ə.`wɛt/. Most lexicographers have preferred to use the former type no [doubt] because it correlates more satisfyingly with the orthography. Yet it can hardly be denied that it tends to suggest a slightly slower and/or more deliberate enunciation than the word ordinarily receives and that the latter version, despite its containing a suggestion of a schwa which can’t be straightforwardly correlated with the word’s orthographical form, satisfies the criterion of sounding not unduly deliberate and producing an effect that probably anyone would regard as normal. Of course it [would] be possible ideally for lexicographers to show both versions of such words [e.g.] strenuous as /`strenjuəs/ and /`strenjəwəs/ but it’s simpler and space-saving to choose one alternative with the understanding that the other is not being simply rejected.
When a syllable comes at the start of a word and ends in a consonant, it usually isn't pronounced with /ə/. Instead, it is pronounced with what is called an "unreduced" vowel. An example of this is the word trombone. Prefixed words are an exception to this rule (see Greg Lee's answer here: Can foreign words be reduced into English morphemes?), but some of them can optionally be pronounced with an unreduced vowel. So for some speakers, /sʌb/ and /səb/ are variant pronunciation options.
Separately from that, some speakers don't distinguish the sounds /ʌ/ and /ə/. In this case, there would be no meaningful difference between /sʌb/ and /səb/.
The last two syllables:
When the vowel sound /u/ comes before another vowel, we may hear what is called a "linking semivowel" after it, meaning, it sounds kind of like /uw/ or, if unstressed, it might sound like /əw/ or /ʊw/. There is no contrast in this context between a pronunciation with or without a "w" sound. Since it is not contrastive in terms of meaning, it makes sense not to include it in a phonemic transcription (which is what slashes technically are used for), so I would prefer the transcription /uənt/. That does not mean that you need to avoid using a /w/ sound here: it's fine as long as you don't overdo it by making it sound as strong as the /w/ found at the start of a syllable in words like "went". The phonetician John Wells writes
Syllable-final sonorants are shorter, more lightly pronounced, than the longer, more deliberate, syllable-initial ones.
There are different pronunciations for the same word in American and British English dictionaries. Meriam Webster is a American dictionary and Wiktionary is mostly a British one. That might be the reason of the variation. An example from Britannica is given below:
"The most obvious difference is the way the letter r is pronounced. In British English, when r comes after a vowel in the same syllable (as in car, hard, or market), the r is not pronounced. In American English the r is pronounced."