How to pronounce terminal "-al"

For American pronunciation, both the Oxford dictionary's American pronunciation and the American Heritage dictionary's leave out the /ə/ in words like little, riddle, mantle, metal that I think of as ending /təl/ and /dəl/, but not words ending in other consonants followed by /əl/, like /nəl/, /bəl/, /kəl/. Merriam-Webster uses /t^əl/, /d^əl/ and /n^əl/, but uses /əl/ after other consonants; the /^əl/ symbolizes the same pronunciation as in Oxford and American Heritage: the /l/ is the nucleus of its own syllable, which contains no vowel.

For British pronunciation, the Cambridge Advanced Learner's Dictionary on-line uses either /l/ and /^əl/ for all these words; I think it depends only on the spelling. I believe these are supposed to symbolize the same sound (no schwa). The online global Oxford dictionary seems to use /(ə)l/ for all of these words, meaning the schwa is optional. So presumably the schwa is supposedly left out in RP.

Going to forvo.com and listening to the pronunciation of words, it seems that Americans generally (not always) do put the schwa in after consonants other than /t/ and /d/, but many (not all) Americans leave the schwa out after /t/ and /d/. I'm not a linguist, so I can't say for sure, but I believe these variations are allophonic, meaning that English speakers will perceive them as having the same phoneme. If you leave out the schwa after /t/ or /d/, it is likely you are unaware you're doing so. Don't worry about pronouncing this "right"; you should be understood just as well either way. Even if you're trying to lose a foreign accent, judging from the pronunciations on Forvo, the variation in both native British and native American speakers is large enough that I don't think anybody will notice this. For example, I leave out the schwa after /k/ and /g/, but put it in after /n/.

In my comment above, I said there were no minimal pairs for /əl/ and syllabic /l/, because they are allophonic, but I think I was wrong. The way I speak, there are several multi-word minimal pairs; for example, metal ion versus met a lion, or mental aberration versus meant elaboration.

There are two allophones of /l/ that occur in English, which may be confusing you. Let's take the word "local" as an example, since you mention it.

The first /l/ is the "light" allophone [l]. This is fairly short, transitioning straight away to the following vowel, and generally behaving in the brief manner that you would expect of a consonant.

The second /l/ is the "dark" allophone [ɫ]. While this has the same basic mouth shape (the tongue is slightly flatter against the roof of the mouth), it has a considerably longer duration. In many ways it functions more like a vowel than a consonant, as is particularly obvious with words like "little" where it is, in effect, the final vowel.

All of the words you give have "-al" as an unstressed final syllable, and they all use [ɫ] there. The vowels in unstressed syllables have a tendency to become a neutral schwa /ə/. This is a short vowel, and in turn tends to shorten in favour of the following /l/, either disappearing entirely or becoming so short it might as well have disappeared.

Exactly how much the schwa disappears seems to be a personal thing; you make one list, @Kosmonaut pronounces them all with a schwa, I pronounce them all without. I don't think there is a general rule, because I don't think there is general agreement.