To your question, "Has anyone experienced genuine "BONE" changes in  ("jaw widened, moved forward, had an upswing") by using mewing, chewing with incisors, molars, or open mouth chewing in their 20s"?
The major facial structure growth/change I've experienced have been in the past 3.5 years (23 - 26) from a very asymmetric posture and barely reaching the mid palate with the tongue to a more even structure where I can comfortably suction my tongue to my front + mid palate and begin mewing. 
After not experiencing much physical growth up to about 21, I wasn't sure if bone structure growth was feasible/pliable after my 20's, but safe to say as a 26.5-year lifespan human, thus far, bone pliability is just as moldable now as it was when beginning my bone structure development at age 23 in 2021. 

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I have. I’m typing this quick apologies for messy grammar.

I’m 24 and have noticeably widened my jaw at the gonion (when viewed from the front) as well as moved my cheekbones forward — to the point where people tell me I look different. I believe I can still make way more gains but I’m shocked at the difference already. I have been playing with mewing for 2-3 years now, but totally unseriously until about 4-6 months ago. 

For the jaw width I think the primary driving factor was actually doing chin tucks, quite extreme ones at that. I suspect that the ramus and gonion hitting the SCM muscle in a chin tuck is actually what causes the widening. I’m not sure though, and this is the area I need most gains on.

For the cheekbones, I literally began doing this technique a week ago where I take a pretty thick towel, bite down on it, and pull it upwards and forwards — to simulate as if I’m tearing a piece of meat in the jungle thousands of years ago lol. Holy sh*t does it work… Literally I feel my maxilla bearing so much tension and I **see** the zygos moved forward it’s insane. Everyone reading must try this at least once to feel what I’m talking about. 

As for mewing, I have only really experienced the sensation that I was making progress from intense hard mewing in a chin tuck position. I’m developing more of a feeling and it seems to be working well, but I still need to work on holding the mewing position all day.

Open mouth chewing was very interesting for me, I felt that when I bite with my molars/premolars (one of these not sure which yet), the force is transferred from my lower teeth to my upper teeth right into my maxilla — it sits in that vertical line and I feel my maxilla feels “fuzzy” akin to it being stimulated with some movement. Overall I think there’s something here, the teeth in contact at this point while you mew helps a lot too to multiply that force.

For the doubtful reader that lost hope... I can assure you all this stuff works even well past 18.