Julie, you kinda lost me. Sorry.
Meiosis becomes, in aggregate, a numbers game with more randomization than not. Even if we could get to the point where we'd be accurate enough to evaluate to a 1cM level (not a given at all, if for no other reason than the assumptions and estimations involved in calculating centiMorgans), you would have ancestors with multiple living descendants, none of whom will carry any detectable DNA from that ancestor. A person can have 20 children and never pass down her entire genome; there is genetic truncation, deprecation at every generation. And without exhuming the ancestor, we would still have to rely on comparing people who have actually tested.
If you flip the numbers and say, hypothetically, that of the 9g-grandparent only 25% of her living descendants still carry any of her DNA, then we're at one-in-four to start. With the intervening 22 birth events you're actually down to a 0.002% chance you and any given 10th cousin carry any matching DNA. But in order to determine that two 10th cousins align back to a specific 9g-grandmother as an MRCA, they both have to share a segment of the same DNA that can be traced back to the ancestor.
To arrive at that theoretical number, take the expected sharing of any one descendant from the ancestor, and then divide it by the number of ancestors at that generation (described in a limited fashion here). For example, we have (again without pedigree collapse) 8 great-grandparents. Any given descendant of one of the great-grandparents would be expected to have around 12.5% of her DNA. Second cousins whose MRCA couple is one set of great-grandparents would have an expected total DNA sharing of about 3.125%. So to get an expected amount of DNA the two cousins would share that came from the same great-grandparent, divide 12.5% by 8 = 1.56%.
Now let's put the numbers on steroids. With no pedigree collapse you would have 2,048 9g-grandparents. Theoretical, even-distribution sharing would mean you would have about 0.048825% of her DNA. If we assume that an entire genome comprises 7,000cM (which splits the difference between the traditional FTDNA 6,800cM model and 23andMe's ~7,200cM), that's roughly 3.4cM.
You and any given 10th cousin descended from the 9g-grandmother would be expected to share about 0.00005% of your DNA...or 0.0035cM. And if we use the formula to see how much the two of you might share from the same 9g-grandmother: 0.048825% ÷ 2,048 = 0.00002384%, which would be equivalent to 0.001669cM. Call it 1,600 total base pairs. With gene linkage in meiosis and linkage disequilibrium, there are very few places on the genome where that might be potentially genealogically meaningful, even with 100X coverage whole genome sequencing.
And, of course, I said my guess was 15 generations as an eventual autosomal threshold. Now I'm thinking I was too optimistic.