You've picked a big topic for your first question on this site! There is a fairly long Wikipedia article about English-language vowel changes before historic /l/; I would say that most of the listed vowel changes are related to the phenomenon of "dark l".
The specifics of how vowels are pronounced before "dark l" vary substantially between accents. Some accents merge certain vowel phonemes before dark l, but the specific vowel that are merged differ by region, or even just from speaker to speaker.
"Dark l" does tend to cause backing of the preceding vowel
A very widespread phenomenon is that "dark l" causes the preceding vowel to be pronounced backer (in phonetic terms). This is probably most noticeable with /u/, since for many speakers that vowel is otherwise fronted/centralized to something that could broadly be transcribed [ʉ]. But before "dark l", /u/ tends to be a truly back vowel that could broadly be transcribed as [u]. This backing also applies to vowels other than /u/, though: for example, for me /ɛ/ and /æ/ are definitely backer when they come right before [ɫ].
Addressing some of your other questions
I don't think a vowel is any shorter before [ɫ] than it would be expected to be before any sonorant consonant (e.g. /n/ or /m/). A sequence of a vowel + [ɫ] will be shorter before a voiceless consonant than in other contexts (e.g. the /ɛl/ in "melt" will have a shorter duration than the /ɛl/ in "meld" or "bell").
- Does an additional glide/vowel appear?
In some accents, including mine, a schwa-like sound tends to be inserted before dark l. It can be syllabic or non-syllabic (sometimes it's unclear to native speakers whether it is a full syllable or not): the extent to which it is present and seems syllabic can be based on the identity of the preceding vowel phoneme. For example, syllabic /ə/ is commonly inserted between /aɪ/ and dark l.
- Is the syllable tenser (requires more effort?)
I haven't yet encountered a definition of "tenseness" that seems plausible as a single phonetic characteristic to me. Whether a vowel is "tense" or "lax" in phonological terms depends on the vowel system of a dialect. Some of the neutralizations that I mentioned above (and that are described in detail at the linked Wikipedia article) could be seen as a kind of tense-lax neutralization before dark l (some similar contexts where we can see neutralization of the tense-lax contrast in some American English accents are before /ŋ/ and before /r/).
- What happens when a dark L becomes a light L?
That depends on the accent. In some accents, such as mine, dark L always stays dark. "Soul and spirit", bowling, schooling, balling, and for me, even monomorphemic trochees like yellow and follow have dark l. I don't know that much about how it works in accents that do change dark L to light L based on the context, but I would guess that the vowel is also lightened in that case.