Learn English – How to spell out short vowel pronunciations

I've always said "Dell fee".

In further support of this, the name was chosen (cite) by one of Delphi's developers, Danny Thorpe, who has a decidedly non-Greek name.

If you look into it further, you'll see the name is a reference to the Oracle at Delphi, which was pronounced in ancient Greek as /ðelˈfi/, using the Ancient Greek IPA (/i/ refers to the "hard e" sound, whereas /ai/ would refer to "hard i"). And according to this footnote, "dell fi" is actually the English corruption.

I watched the first Youtube video you referenced, done by Rachel, and I think several of the observations are good, but overall, it misses the point. The important difference between dark l and neutral l in English is retraction of the tongue body for the dark l. (I'm calling the non-dark l "neutral", because I don't think l in English is truly light -- just sometimes not dark.)

The position of the tongue tip doesn't matter. When the body of the tongue is pulled back, that may affect the position of the tongue tip, but that's just because the tongue tip and the tongue body are connected -- they're both part of the tongue. It's the retraction of the tongue body that makes the l dark, whether or not there is any difference in the position of the tip.

Rachel talks about the pronunciation of "real" and observes, correctly, that there is an extra vowel sound that intrudes between the [i] and the [l]. But that is not what makes the [l] dark. When the tongue body is pulled back for the dark l, the body of the tongue has to move from front to back, since it was pushed to the front to make the [i] vowel, and it has to be pulled back for the dark l. To get from one place to the other, the tongue body has to move through an intermediate central position, and this is what Rachel hears as an extra vowel sound between the [i] and [l]. This central glide is predictable if you have a correct idea about how dark l is articulated.