Sorry to disappoint but you can't grow an Osakazuki from seed :-) You can grow a generic Acer palmatum seedling that may resemble Osakazuki, but any named forms must be propagated clonaly or asexually (graftings or cuttings). And Japanese maples grown from seed may or may not look anything like the mother plant, especially if there are other varieties nearby that could contribute to cross pollination.

As to the speed of germination, some JM seeds just take longer than others. As one very reliable source states: "You just can’t just plop them in the ground. Harvesting must be done in
the fall at the specific time. And this is followed by a whole regimen of
cold stratification in your refrigerator for several weeks to several
months before you are able plant them. Even then your success rate will
likely be slim. Plant 100 stratified seeds you may get a 10-20 to
germinate (some may take up to two years to do so)."

Maple seeds picked while fully developed yet still soft come up more readily than those gathered and planted after becoming hard and brown. If your seeds from 'Osakazuki' had become hard whereas the other two happened to have been gathered while still soft that could account for differences in response right there.

As mentioned, even though a percentage of seedlings of named Japanese maple cultivars may resemble the parents these really shouldn't be called by the same names. This produces a muddling effect where different people have different clones they are all calling by one name. Even though annual flowers and vegetables are maintained as seed strains with perennial plants including trees and shrubs the convention is that named selections are clonal and therefore strictly uniform. Otherwise consistency becomes lost - picture what it would be like if multiple different cars were all being called the same make and model.

Only 2 maples in my garden regularly provide seeds,Bloodgood&Osakazuki.I have to admit I tend to neglect them so they never get very old.The vast majority are green and at a young age I can't tell them apart and I haven't seen the characteristic amoneum leaf shape on either.I assume if this will develop,it'll come later after 2years.
Anyway,regarding germination they both seem quite reliable,I cold stratify by leaving outside all winter after a pre-soak.They are harvested just when the wings start to brown.

Yep thanks Houzi, that gives me hope :)

I fully understand the expected genetic variation from seeds. Of about 10 seiryu seedlings 9 had the regular palmatum leaf shape. Only 1 had a leaf shape similar to seiryu, but that one died!

I just hope i can at least get 1 with the similar amoenum shape to osakazuki

120 plus days now and still nothing :|

I've taken about 5 out into warmer conditions to see if something happens.