That's correct, the tip of the tongue touches the inner surface of upper teeth when pronouncing the Russian hard "L".
I've never heard your pronunciation, but as to me, I doubt you can "dentalize it too much"
Agree.
Yes. And one does not need to have "very good ears" for that. When I heard Russian pronunciation of some Americans, I always noticed their "L" sounded "semi-soft" to me when followed by a vowel. A word-final "L" is usually OK, but vowels somehow affect it more.
The reason for that is the Russian "hard L" is velarized which strengthens the contrast with its soft counterpart. And Russians sub-consciously hear it "soft" when it is not velarized. That's what happens to French, German, Spanish, Italian L's: they are neither palatalized nor velarized, which makes Russians perceive them as "soft".
I've just tried to pronounce the velarized Russian "hard L" using the alveolar tongue position, and I'd say it is physically possible, but very difficult. It is hard to keep the correct "spoon-like" shape of the tongue of the velarized L with the alveolar tip position.