In most of these cases the final B is itself silent but elongates the previous wovel for me. "dumb" is pronounced longer than "dum", and it's the U that gets elongated. The difference is subtle, but my native tongue has "long wovels" so I'm trained to detect the difference.

That's true for the -omb words, but I've never heard anyone elongate the 'u' in any of the -umb words, at least not in Canadian English.

Paljon suomalaisia täällä?

Empiirisesti ehkä muutama tuhannesta.

Wait, what? None of those have a silent B, at least in my region. Are there places that pronounce the same as womb as "woom"?

Can I ask what region you live in? Pronouncing <womb> as /wuːm/ is the dominant pronunciation in every English speaking country, so much so that I can't recall ever hearing it differently, and I'm surprised you've never heard it. It's not a recent change either, the /b/ disappeared somewhere before the 14th century.

New England. I've never paid enough attention when traveling to notice (those words aren't exactly the most common to hear), but when I say "woom" my lips end closed, and when I say "womb" my lips end opened after closing. The b isn't "voiced", per se, but it does change the way the word sounds to me.

And to be fair, I am aware New England English is its own distinct thing so I might have grown up in a bubble:

https://en.wikipedia.org/wiki/New_England_English

NE Ohio and I always pronounced the b too (akin to New England I guess)