The probability also depends on the other parents genotype. If the other parent has a genotype of RR then the probability is 0%. If the genotype is Rr it is 25%. And if the genotype is rr it is also 25%.
10 chromosomes. After telophase and cytokinesis, the new daughter cells will each have 10 chromosomes, which is identical to the parental cell. Human cells have 23 pairs of chromosomes.
In order to "invest" in later stages to get more energy per glucose molecule. It could also be because that is how it is done, it wouldn't be as efficient unless these stages were performed.
hope this helps!